Embryos as Missing Lin\s 135 



distinct group. While the three classes of moUusks have a 

 great deal in common, they all differ from other branches 

 so much that it is difficult to find convincing affinities. If, 

 however, representatives of these classes are studied from the 

 point at which all animals are alike in structure, namely 

 the one-celled or t^^ stage, significant facts appear. After 

 the segmentation of the sand-worm's Qg^ has passed the two- 

 layered stage, certain regions of the cell mass put out vi- 

 bratile hairs or cilia by means of which the embryo can 

 swim (Fig. 36). There is developed a free swimming larva 

 called the trochophore which resembles no adult animal we 

 know, except in a general way the ** comb bearers," some- 

 times mistaken for jellyfish. The resemblance here is very 

 general: there is a sac-like body with rows of cilia extend- 

 ing along the side between the poles. As the trochophore 

 of the worm is closely watched in its development, we 

 see changes which hour by hour elongate the body in the 

 region farthest from the mouth. The ring of cilia becomes 

 crowded forward and the cilia themselves eventually dis- 

 appear. The elongated hind part becomes constricted into 

 rings or segments, each of which has a portion of 

 the " body cavity " and a segment of the food tube, as 

 well as portions of the main blood vessels and of the nerve 

 strands. 



We do not know why the worm develops from the egg 

 in this roundabout way; but the facts are clear. If we fol- 

 low the development of a clam's Qgg or a snail's €gg we find 

 again after the two-layer stage the formation of a trocho- 

 phore. With the hundreds of ^ species of mollusks and of 

 segmented worms living in the ocean, there are at times 

 millions of trochophores floating about, and it is impossible 

 for most observers to know the larvse of different species 

 apart, or even the larvae of worms from the larvae of mol- 

 lusks. Different as the clam is from the worm, both are 

 alike not only in the one-celled stage but in the trochophore 

 stage. From this point on the individual oyster or clam 

 follows a different path from that of the worm. The iden- 



