328 



THE LARVAL FORM. 



end of the prae-oral lobe (fig. 146), and finally the head becomes relatively 

 reduced as compared with the body, and gives rise to the simple head of 



FIG. 146. POLYGORDIUS LARVA. (From Alex. 

 Agassiz.) 



the fully formed worm (fig. 148). The two ciliated bands disappear, the 

 posterior vanishing first. The ciliated band at the hind end of the body 

 also atrophies ; and just in front of it the ring of wart-like prominences used 

 by the adult to attach itself becomes developed. 



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 larvae, but persistent in the 

 adult Polygordius, Saccocirrus, Poly- 

 ophthalmus, 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 

 Chaetopoda ; 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 FlG _ ^ POLYGORDIUS LARVA. 

 notice are the excretory organs. (From Alex. Agassiz.) 



The essential points in the above development of Polygor- 

 dius are (i) the gradual elongation and corresponding segmenta- 

 tion of the post-cephalic part of the body ; and (2) the relative 

 reduction in size of the prae-oral lobe and its conversion to- 

 gether 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 



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



