272 



THE LARVAL 



At the sides of the head there is 

 formed a pair of ciliated pits, also found 

 by Hatschek in the embryo of Criodrilus, 

 and characteristic of many Chaetopod lar- 

 vae, but persistent in the adult Poly- 

 gordius, Saccocirrus, Polyophthalmus, etc. 

 They are perhaps the same structures as 

 the ciliated pits in Nemertines. 



During the external changes above 

 described, by which the adult form of 

 Polygordius is reached, a series of in- 

 ternal changes also takes place which are 

 for the most part the same as in other 

 Chsetopoda ; and do not require a detailed 

 description. The nervous 1 and muscular 

 systems have precisely the normal de- 

 velopment. The division of the meso- 

 blast into somites is not externally in- 

 dicated. The organs most worthy of 

 notice are the excretory organs. 



The essential points in the above 



development of Polygordius are (1) the gradual elongation and 

 corresponding segmentation of the post-cephalic part of the body; 

 and (2) the relative reduction in size of the pras-oral lobe and its 



FlG. 147. POLYGOKDIUS LAKVA. 



(From Alex. Agassiz.) 



FIG. 148. POLYGOBDIUS LARVA. (From Alex. Agassiz.) 



conversion together with the oral region into the head ; (3) the 

 atrophy of the ciliated bands. The conversion of the larva into the 

 adult takes place in fact by the intercalation of a segmented region 

 between a large mouth-bearing portion of the primitive body and a 

 small anus-beariug portion 2 . 



The general mode of development of chsetopod larvae is similar 

 to the above except in details, which are however no doubt often of 

 great importance. The history of the larvae may be conveniently 

 treated under three heads. (1) The form of the primitive unseg- 

 mented larva; (2) the arrangement of the cilia on the unsegmented 

 larva, and on the larva at later stages; (3) the character of the meta- 

 morphosis and the development of the permanent external organs. 



1 The structure of the ventral cord in the adult requires further elucidation. 

 a For Semper's view as to the intercalation of segments in the cephalic region, vide 

 note on p. 276. 



