Cross references:  Bilateria   Flat Worms   Deuterostomes      

Protostome (Wiki) 

"Protostomia (from the Greek: mouth first) are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers. The major distinctions between deuterostomes and protostomes are found in embryonic development

In (protosomes), the embryo forms a dent on one side, the blastopore, which deepens to become the archenteron, the first phase in the growth of the gut (and therefore the mouth). In deuterostomes, the original dent becomes the anus while the gut eventually tunnels through to make another opening, which forms the mouth.

Current molecular data suggest that protostome animals can be divided into three major groups:

as well as a number of minor taxa of basal or ambiguous affinity." 

Bilateria (Wiki)
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. "  "For the most part, Bilateria have bodies that develop from three different germ layers, called the endoderm, mesoderm, and ectoderm. From this they are called triploblastic. "  "... Bilateria have complete digestive tracts with separate mouth and anus."  "The first evidence of bilateria in the fossil record comes from trace fossils in Ediacaran sediments, and the first bona fide bilaterian fossil is Kimberella, dating to 555 million years ago.

Triploblasty (Wiki)

"Triploblasty is a condition of the blastula in which there are three primary germ layers: the ectoderm, mesoderm, and endoderm. The germ layers form during gastrulation of the blastula. Additionally, the term may refer to any ovum in which the blastoderm splits into three layers. All higher and intermediate animals (from flat worms to humans), arise from a triploblastic ovum. Triploblastic organisms are organisms which evolve from such an ovum. Generally speaking, triploblastic organisms belong to the Bilateria subregnum. Triploblastic organisms generally possess bilateral symmetry which is where the clade Bilateria takes its name. Simpler diploblastic animals such as cnidaria (which includes jellyfish, corals and hydra) possess two germ layers."

Caenorhabditis elegans (Wiki)   

"Caenorhabditis elegans ... is a free-living, transparent nematode (roundworm), about 1 mm in length,[2] which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model organism."

Pheromonal stimulation of spawning release of gametes by GRH in the chiton (PubMed) 
Only abstract available online. 
The chiton Mopalia sp., a mollusc, was exposed to various dilutions of gonadotropin releasing hormone (GnRH) in sea water to determine whether this peptide is capable of acting as a pheromone that could stimulate release of ripe gametes (spawning). Two of the peptides, lamprey GnRH-1 and tunicate GnRH-2, had this action at a higher concentration (1.0 mg/L) but dilutions to 50 microg/L no longer were effective. Three other GnRHs: lamprey GnRH-3, tunicate GnRH-1, and a modified chicken GnRH-2, had no such action under the same test conditions. Since the spawning response could be produced by some GnRHs and not by others, it would appear that some kind of molecular recognition is involved, possibly by specific binding to a receptor. In earlier preliminary experiments tunicate GnRH-2 rapidly stimulated gamete release in a hemichordate, Saccoglossus. Thus it is suggested that GnRHs, in at least some invertebrates, may function as pheromones, serving to stimulate simultaneous spawning of individuals in a population of animals, and in this way assure more successful fertilization in species that must release their gametes into the water in which they live."  
    My comment
Gonadotropin releasing hormone (GnRH) as possible pheromone.  First mention of a hormone. 

Chiton (Wiki)    
    "Chitons are small to large, primitive marine molluscs in the class Polyplacophora. There are 900 to 1,000 extant species of chitons in the class, which was formerly known as Amphineura.[3]
    These molluscs are also sometimes commonly known as sea cradles or "coat-of-mail shells". They are also sometimes referred to more formally as loricates, polyplacophorans, and rarely as polyplacophores.