THE EVIDENCE FROM DEVELOPMENT. 



231 



coasts are good examples. Hence we conclude that the Brachiopods 

 present us with a group, which has sprung from a worm-like stock, 

 along with the sea-mats, thus showing us the possibility of higher 

 molluscs having had a similar origin. 



The early history of the worms themselves belonging to the 

 Annulose type forms a concluding phase in these investigations 

 into the history of the 

 Molluscan race. If we 

 study the development 

 of one of the true sea- 

 worms, such as Arenicola (Fig. 155) or Nereis (Fig. 156), we shall find 

 a striking reproduction of some features with which our molluscan 

 researches have already made us familiar. The young worm (Fig. 

 157) makes its first appearance as an active free- swimming, barrel- 

 shaped body, provided with cilia, disposed in various fashions, in 



different groups of the class. 

 Thus, in some embryos (B, C) 

 there is a first band of cilia 

 around the body in front of the 

 mouth, a second band exists 

 at the opposite extremity, and 

 tufts of these cilia may also 

 be developed at the extreme 

 front of the body. In other 

 cases numerous bands of cilia 

 encircle the body at its middle 

 portion only (C); whilst a third 

 set of cases exists where a broad zone of cilia occupies the middle 

 region, with or without a tuft at the head-extremity. Out of such 

 larval forms, the young worms are gradually developed, the head and 

 front segments appearing first 

 in the order of growth. Cer- 

 tain of those sea- worms which, 

 like Serpula (Fig. 158), live in 

 tubes of lime or other matters 

 which they fabricate from the 

 minerals of the sea- water, 

 possess a development equally 

 characteristic with that of 

 their free-living neighbours. 

 In the larvae of these tube- 

 dwelling WOrmS (Fig. 1157, FIG. 157. DEVELOPMENT OF WORMS. 



A, D), the head is provided 



with cilia, disposed chiefly in two rings, one at either extremity 



of the body. Soon tentacles are developed from the head portion, 



FIG. 156. NEREIS. A Marine Worm. 



