330 



THE LARVAL FORM. 



A good example of a larval form with a small prae-oral lobe is afforded by 

 Capitella, which is figured by Clap, and Metsch. (No. 336, PI. xvn. fig. 2). 

 The imperfect development of the prae-oral lobe is also generally character- 

 istic of the Oligochaeta. The persistence of a relatively large prae-oral lobe 

 for so long a time as in Polygordius is very unusual. 



The arrangement of the cilia in Chaetopod larvae has been 

 employed as a means of classifying them. Although a classifi- 

 cation so framed has no morphological value, yet the terms 

 themselves which have been invented are convenient. The 

 terms most usually employed are Atrochae, Monotrochae, 

 Telotrochae, Polytrochae, Mesotrochae. The polytrochse 

 may again be subdivided into Polytrochae proper, Nototrochae, 

 Gasterotrochae, and Amphitrochae. 



The atrochae contain forms (fig. 139) in which the larva is at 

 first coated by an uniform covering of cilia, which, though it 

 may subsequently disappear from certain areas, does not break 

 up into a series of definite bands. 



The monotrochae or cephalotrochae are larvae in which only a 

 single prae-oral ring is developed (fig. 150 B). 



In the telotrochae there is 

 present a prae-oral and a post- 

 oral, i.e. peri-anal ring (fig. 150 

 A) ; the latter sometimes hav- 

 ing the form of a peri-anal 

 patch. 



The polytrochae are seg- 

 mented larvae with perfect or 

 imperfect rings of cilia on the 

 segments of the body usually 

 one ring to each segment 

 between the two characteristic 

 telotrochal rings. When these 

 rings are complete the larvae 

 are polytrochae proper, when they are only half rings they are 

 either nototrochae or gasterotrochae. Sometimes there are both 

 dorsal and ventral half rings which do not however correspond, 

 such forms constitute the amphitrochae. 



In the mesotrochae one or two rings are present in the middle 

 of the body, and the characteristic telotrochal rings are absent. 



FIG. 150. Two CH./ETOPOD 

 (From Gegenbaur.) 



o. mouth ; i. intestine ; a. anus ; 

 v . prse-oral ciliated band ; w. peri-anal 

 ciliated band. 



