Cross references:  Hemichordate Hormones      Hemichordate Nerves    
Hemichordate Neurotransmitters      Hemichordate Genomics    
Hemichordate Larva (Tornaria)     Deuterostomes   Chordates    

Hemichordata (Wiki) 

    "Hemichordata is a phylum of worm-shaped marine deuterostome animals, generally considered the sister group of the echinoderms. They date back to the Lower or Middle Cambrian... "   
Hemichordata are divided into two classes: the Enteropneusta,[1] commonly called acorn worms, and the Pterobranchia

Acorn worm (Wiki) 
NOTE:  Much of this Wikipedia page is drawn from their reference [3] , which I have discussed more thoroughly in the My comments section, below.  Since there is information in the reference that I find interesting but which Wikipedia has not included in their page, I've added the missing parts.  These are enclosed in {  ... } brackets.     
The Acorn worms or Enteropneusta are a hemichordate class of invertebrates, closely related to the echinoderms.[1]

There are about 90 species of acorn worm in the world, the main species for research being Saccoglossus kowaleskii.
All species are infaunal benthos that either may be deposit feeders or suspension feeders. Some of these worms may grow to be very long; one particular species may reach a length of 2.5 metres (almost eight feet), although most acorn worms are much, much smaller.
Acorn worms move only sluggishly, using ciliary action.  {Burrowing or movement within the burrow is accomplished largely by the proboscis which is lengthened and contracted by} peristalsis .[3]
A plexus of nerves lies underneath the skin, and is concentrated into both dorsal and ventral nerve cords. While the ventral cord runs only as far as the collar, the dorsal cord reaches into the proboscis, and is partially separated from the epidermis in that region. This part of the dorsal nerve cord is often hollow, and may well be homologous with the brain of vertebrates. In acorn worms, it seems to be primarily involved with coordinating muscular action of the body during burrowing and crawling.[3]
Acorn worms have no eyes, ears or other special sense organs, except for the ciliary organ in front of the mouth, which appears to be involved in filter feeding and perhaps taste (3). There are, however, numerous nerve endings throughout the skin.[3]
An interesting trait is that its three-section body plan is no longer present in the vertebrates, except for the anatomy of the frontal neural tube, later developed into a brain which is divided into three main parts. This means some of the original anatomy of the early chordate ancestors is still present even if it is not always visible.
  One theory is that the three-part body originates from an early common ancestor of all the deuterostomes, and maybe even from a common bilateral ancestor of both the deuterostomes and protostomes"    
Acorn worms are dioecious, having separate biological sexes, although at least some species are also capable of asexual reproduction. They have paired gonads, which lie close to the pharynx and release the gametes through a small pore near to the gill slits.  
    The female lays a large number of eggs embedded in a gelatinous mass of mucus, which are then externally fertilised by {sperm emitted from nearby males that are apparently stimulated by the presence of the released eggs} before water currents break up the mass and disperse the individual eggs.[3]
    In most species, the eggs hatch into planktonic larvae with elongated bodies covered in cilia. In some species, these develop directly into adults, but in others, there is a free-swimming intermediate stage referred to as a tornaria larva. These are very similar in appearance to the bipinnaria larvae of starfishes, with convoluted bands of cilia running around the body. Since the embryonic development of the blastula within egg is also very similar to that of echinoderms, this suggests a close phylogenetic link between the two groups.[3]
My comments
    1.    Although not stated explicitly, the male apparently does not leave his burrow to fertilize the eggs.
  The description, above, doesn't make him sound very mobile.  This is quite different from the  Amphioxus .      See:   Amphioxus Behavior.   
    This point is important to me.  Although I've gotten bogged down with physiology and evolution, my real interest is psychology, and the aspect of psychology that I'm most interested in is behavior.  If the male acorn worm left his burrow to fertilize the eggs, that would be an early manifestation of behavior and, therefore, would become a major focus of my attention.  However, as far as I can tell, that sort of behavior begins with the  
Amphioxus  .  I suppose you could call just releasing their sperm into the surrounding water "behavior", but you have to draw the line somewhere, and I've chosen to draw it between the very passive acorn worm and the occasionally much more active amphioxus.    
    2.  The males are probably stimulated to release their sperm by gonadotrophin releasing hormone (GnRH) acting as a pheromone. 
Hemichordate Hormones .   
    3.  Wikipedia's reference 
[3] , above is:   
Barnes, Robert D. (1982). Invertebrate Zoology.  pp. 1018–1026. ISBN 0-03-056747-5.
  I wasn't able to find a 1982 edition, but the library did have the 1963, 1968, 1974, 1980 and 1987 editions, and I looked at all of them.  They're all very similar, and none of them provides references for any of their specific statements.  The additions I've made, above, to the Wikipedia page are from the 5th edition, copyright 1987.  
    4.  In addition to their apparently immobile mating style, the acorn worms also do not seem to be able to escape quickly when disturbed.  In contrast, I have the impression that the amphioxus is able to escape quite vigorously.   

Pterobranchia - Wikipedia 
Pterobranchia is a clade of small worm-shaped animals. They belong to the Hemichordata, and live in secreted tubes on the ocean floor. Pterobranchia feed by filtering plankton out of the water with the help of cilia attached to tentacles. There are about 30 known living species in the group."   

Temporal range: Cambrian stage 3–Recent[1]

Cephalodiscus dodecalophus McIntosh.png

    "Studies under an electron microscope have suggested that pterobranchs belong to the same clade as the extinct graptolites,[2][3] and phylogenetic analysis suggests that the pterobranchs are living members of the graptolite clade.[4] 
Introduction to the Hemichordata  
Short, full-length, article available online for free. 
    " Of the three classes of hemichordates, the most familiar living ones are the Enteropneusta, the acorn worms."   
    "They are slow burrowers, using the proboscis to burrow through sediment, and may either deposit feed (consume sediment and digest the organic matter, rather like earthworms in soil) or suspension feed (collect suspended particles from the water). Some of these worms may be very large; one species may reach a length of 2.5 meters (almost eight feet), although most are much smaller."    
     "The second living class is the Pterobranchia, an obscure group with only about 20 living species. Pterobranchs are very different from acorn worms; they form colonies in which the individuals are interconnected by stems, or stolons. Individuals, or zooids, are often less than 1 millimeter long. The proboscis is not elongated, as it is in acorn worms, but shield-shaped. The second division of the body bears a pair of branched tentacles that collect small food particles from the water. There is only one branchial opening.  
    Most strikingly, almost all pterobranch species create and live within a network of tubes, the coenecium. These tubes are made up of the protein collagen, secreted by special glands in the proboscis. Yet similar larvae and a similar tripartite body plan unite the enteropneusts and pterobranchs.
What is now considered a third class of hemichordates, the Graptolithina or graptolites ... are common fossils in Ordovician and Silurian rocks ...

Evolution of the chordate body plan: New insights from phylogenetic analyses of deuterostome phyla
from the abstract    
The deuterostome phyla include Echinodermata, Hemichordata, and Chordata. Chordata is composed of three subphyla, Vertebrata, Cephalochordata (Branchiostoma), and Urochordata (Tunicata).
Hemichordates include two distinct classes, the enteropneust worms and the colonial pterobranchs. Most previous hypotheses of deuterostome origins have assumed that the morphology of extant colonial pterobranchs resembles the ancestral deuterostome. We present a molecular phylogenetic analysis of hemichordates that challenges this long-held view.
We used 18S rRNA to infer evolutionary relationships of the hemichordate classes Pterobranchia and Enteropneusta. Our data show that pterobranchs may be derived within enteropneust worms rather than being a sister clade to the enteropneusts. The nesting of the pterobranchs within the enteropneusts dramatically alters our view of the evolution of the chordate body plan and suggests that the ancestral deuterostome more closely resembled a mobile worm-like enteropneust than a sessile colonial pterobranch.
from the HTML   
    "Adult enteropneusts exhibit some chordate characteristics, namely pharyngeal gill pores, a stomochord, and an endostyle-like structure in the pharynx" 
    "If chordate-like features of enteropneusts come from similar developmental and or molecular pathways, then similar structures are the result of common origins rather than convergence." 

A phylogeny of the hemichordates based on morphological characters  
PDF no longer available online for free.   
A comprehensive review of literature on all 15 genera constituting the phylum Hemichordata resulted in a morphological matrix of 105 characters. The echinoderms, tunicates, cephalochordates, and vertebrates were included in the analysis, and the cnidarians, polychaetes, and sipunculids were employed as outgroup taxa.  
    The consensus tree supported the traditional view of a monophyletic Hemichordata, Echinodermata, Ambulacraria, and Chordata. The enteropneust families Spengelidae and Ptychoderidae were each monophyletic and sister-taxa, but there was no resolution among the family Harrimaniidae.  
    A detailed sensitivity analysis provided  
        (i) tree lengths of competing evolutionary hypothesis and
        (ii) a test of monophyly of groups under a variety of evolutionary models.
    It is argued that the ancestral deuterostome was a benthic vermiform organism with a terminal mouth and anus, well-developed circular and longitudinal muscles, a simple nerve plexus with little sign of regionalization, a pharynx with gill slits and collagenous gill bars, a cluster of vacuolated cells with myofilaments, produced iodotyrosine, and displayed direct development. The pterobranchs have lost many of these features as a consequence of evolving a small body size and living in tubes, but these features exist in present-day enteropneusts, suggesting that they are a plausible model for the proximate ancestor of deuterostomes."      

Hemichordates and the origin of chordates
Only abstract online.  I got the PDF from the library.   
from the abstract   
    "Hemichordates, the phylum of bilateral animals most closely related to chordates, could reveal the evolutionary origins of chordate traits such as the nerve cord, notochord, gill slits and tail.  
    The anteroposterior maps of gene expression domains for 38 genes of chordate neural patterning are highly similar for hemichordates and chordates, even though hemichordates have a diffuse nerve-net. About 40% of the domains are not present in protostome maps.  
    We propose that this map, the gill slits and the tail date to the deuterostome ancestor.    
    The map of dorsoventral expression domains, centered on a Bmp–Chordin axis, differs between the two groups; hemichordates resemble protostomes more than they do chordates. The dorsoventral axis might have undergone extensive modification in the chordate line, including centralization of the nervous system, segregation of epidermis, derivation of the notochord, and an inversion of organization."  
from the PDF   
    "The notochord, dorsal hollow nerve cord, gill slits and a post-anal tail are phylotypic traits of chordates. Less prominent are the endostyle/thyroid, the pituitary, left–right asymmetries, and the inverse dorsoventral organization of chordates relative to that of protostomes."  
    "Enteropneusts burrow with the muscular proboscis, and move within the burrow by the action of cilia and muscles of the body wall."    
    "Hemichordates have no dorsal hollow nerve cord.  Bullock [30] and Knight-Jones [31] silver-stained the nervous system and found an epidermal nerve net.  Nerves are finely mixed with epidermal cells over the entire body surface. Pan-neural genes sox2/3, elav and musashi/nrp are expressed pan-ectodermally [25..].  Axons extend into a dense basiepithelial mat, some synapsing locally and some extending into thick axon tracts at the dorsal or ventral midlines [30,31,32].     
    These tracts do not constitute a central nervous system; they lack neural cell bodies and hence neurogenesis, and they lack interneurons and hence information processing."    

Figure 1:  Saccoglossus kowalevskii, a direct-developing enteropneust hemichordate of the US Atlantic coast.  
    (a) Adult female. Note the white proboscis, orange collar, tan pharynx and gut, and grey ovaries. Length: 10–18 cm.  
    (b) Juvenile, a week after hatching, two weeks after fertilization of the egg. Length: 1 mm  
    (c) The ‘notochord’, so-called by Bateson (see text), now called the stomochord, located between the proboscis and collar. Shown in sagittal section. Redrawn from Bateson [5].   

    "Although hemichordates and chordates differ anatomically, their body plans are similar in the anteroposterior dimension regarding the topology of the domains of expression of 32 genes [25,29], chosen for their importance in neural patterning. They encode transcription factors and signaling proteins.  
    Genes expressed in the forebrain of chordates are expressed in the prosome of hemichordates.  
    Those in the midbrain of chordates are expressed in the mesosome and anterior metasome of hemichordates, stopping at the first gill slit.   
    Those in the chordate hindbrain and spinal cord are expressed in the hemichordate metasome, entirely posterior to the first gill slit [25].  
    Finally, those in the chordate tail are expressed in the hemichordate post-anal extension, mentioned above."  
    "Presumably, the deuterostome ancestor had this detailed map."    
    "About 60% of the domains are shared with protostomes, indicating their likely presence in the bilateral ancestor."  
    "Although hemichordates and chordates share this map, they develop different morphologies: a diffuse versus a centralized nervous system.  
    The map is clearly more ancient and conserved than are the particular anatomies and physiologies that develop from particular domains in different lineages of animals.  Two differences stand out.  
    First, domains encircle the hemichordate body, each as a band, whereas in chordates they are patches within the neural ectoderm. This correlates with the nervous systems; in hemichordates it encircles the body, whereas in chordates it is centralized within a subregion of ectoderm.  
    Second, most of these genes are expressed only in the hemichordate ectoderm, whereas in chordates they are also expressed in the mesoderm, such as Hox genes in somites."    
    "Chordate evolution, we suggest, entailed little or no change of domain organization from that already present in the anteroposterior axis of the deuterostome ancestor.  Gill slits and the post-anal tail might be ancestral deuterostome traits of this conserved dimension."   
    "Although the hemichordate nervous system is diffuse, it is extensively patterned." 

Man is but a worm: chordate origins
Only abstract available online.  I got the PDF from the library.   
from the abstract     
Recent developmental gene expression data has shown that the chordates use similar gene families and networks to specify their anterior-posterior, dorsal-ventral and left-right body axes.    
    The anterior-posterior axis is similarly established among deuterostomes and is determined by a related family of transcription factors, the Hox gene clusters and Wnt signaling pathways.
    In contrast, the dorsal-ventral axis is inverted in chordates, compared with other nonchordate invertebrates, while still determined by expression of BMP signaling pathway members and their antagonists.    
    Finally, left-right asymmetries in diverse deuterostomes are determined by nodal signaling.

My comments
    1.  This image shows that the nerves cross the midline (dicussate).  This also happens in humans.  See: 
Hemichordate Nerves   for an enlarged picture that includes labels. 
    2.  I'm very interested in what is called 'cerebral laterality'
.  The statement:
... left-right asymmetries in diverse deuterostomes are determined by nodal signaling" indicates that it goes back a long way.   (See:  Human Asymmetry .)
    3.  I obviously need to learn more about 'nodal signaling'. 

Work for the future:     
    Since there's a broad range of information out there that may be of interest to you but which is not directly related to psychology, I'll give a count of the NRC, PubMed and BHL articles for each of our ancestors. 

Additional articles re: hemichordates: 
    NRC: 25 
    PubMed: 85
    BHL: 16  (28 references, 0 reviewed)

Biodiversity Heritage Library (BHL): Name Search - "Hemichordates 

    Since there are so many of these links, I'll be coming back later to look at them. 

Short paragraph, but diagram @ 'Fig. 4', not yet located: