285 ''^f ANIMAL KINGDOM 



a pair of longitudinal cords that later become metameric. In both phyla 

 tlie jKiired nature ol the ventral cord often disappears by fusion, pro- 

 ducing a single atlult nerve cord. 



Thus, the early development of these forms indicates a close re- 

 lation between annelids and molluscs, whereas later development and 

 adult morphology indicates a close relation between annelids and 

 arthropods. Hence, the three phyla are considered to form a natural 

 group within the eucoelomates. 



131. The Trochophore Larva 



The trochophore larva has been the subject of a considerable amount 

 of embryological research. In a given species the cleavage pattern from 

 egg to trochophore tends to follow an exact pattern (which is somewhat 

 less exact in those with much yolk). This pattern is termed a cell lineage. 

 The cell lineages of some of the aschelminthes have been described 

 previously (p. 240). These patterns differ in detail from species to species 

 but are similar in many general features. A comparison of cell lineages 

 in annelids and molluscs reveals that the patterns of development are 

 as similar as the results, i.e., the trochophores not only look alike, but 

 develop in similar ways. 



The trochophore (Fig. 15.10 A) is biconical, with a ring of cilia, 

 the prototroch, around the equator. At the upper apex there is usually 

 a sensory apical organ bearing a tuft of cilia. Brain rudiments are usu- 

 ally evident beneath the apical organ. The mouth is just beneath the 

 prototroch and the anus is near the lower apex. Often (especially if 

 yolk is plentiful) the digestive tract is less well developed than shown 

 here; an intestine and anus may be lacking at this stage of development. 

 The mesoderm is a pair of undifferentiated masses in the lower cone, 

 lying beside a pair of protonephridia that develop from the ectoderm. 

 At this early stage of development the trochophore lacks a coelom; its 

 body is composed primarily of an outer ectoderm with ectodermal de- 

 rivatives such as nervous tissue and scattered ectomesodermal elements, 

 and an inner endoderm forming a gut. 



If cell lineage is followed from the 16-cell stage to the trochophore 

 (Fig. 15.11) in a number of species, it is found that in general the 

 upper cone and prototroch develop from the first quartette (upper eight 

 cells). Of these the upper four cells become the apical organ and most 

 of the cone surface, and the low^er four cells become the prototroch and 

 the lower part of the upper cone surface. Most of the surface of the 

 lower cone is derived from the second quartette (middle four cells). The 

 four large cells become a part of the ectoderm between the mouth and 

 anus (this portion is formed by the cells of the third quartette, which 

 separate from the large cells at the next division), and all of the meso- 

 derm and endoderm. The mesoderm develops from one of these cells 

 while the endoderm comes mostly from the other three. This general 

 pattern of development is found in both the annelids and the molluscs. 



An interesting problem in embryology is whether or not particular 

 cells are able to develop into structures other than those they become 



