CH^ETOPODA. 



273 



FIG. 149. LARVA OF 

 PHYLLODOCE. (From 

 Alex. Agassiz.) 



A larva similar to the Polygordius larva with a greatly developed 

 prae-oral lobe is widely distributed amongst the Annelids. 



An almost identical form is that of Nepthys 

 scolopendroides (Claparede and Metschnikoff, No. 

 336); that of Phyllodoce (tig. 149) is also very 

 similar, and that of Saccocirrus (Metsch. and Clap. 

 No. 336, PI. xni. fig. 1), a very primitive form most 

 nearly related to Polygordius, clearly belongs to the 

 same type. Many other larval forms, such as that 

 of Spio fuligmosus (Metsch. and Clap. No. 336), Tere- 

 bella, Nerine, etc., also closely approach this form. 



Other really similar forms at first sight ap- 

 pear very different, but this is mainly owing to 

 the fact that their prae-oral lobe never attains 

 a considerable development. Its smallness, though obviously of no 

 deep morphological significance, at once produces a very different 

 appearance in a larva. 



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 prse-oral lobe is also generally 

 characteristic of the Oligochreta. The persistence of a relatively large 

 prte-oral lobe for so long a time as in Polygordius is very unusual. 



The arrangement of the cilia in Chsetopod larvaa has been 

 employed as a means of classifying them. Although a classification 

 so framed has no morphological value, yet the terms themselves which 

 have been invented are convenient. The terms most usually employed 

 are Atroclise, Monotrocha?, Telotrochae, Polytrocha3, Mesotro- 

 cha3. The polytrocbse may again be subdivided into Polytrochai 

 proper, Nototrochas, 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 subse- 

 quently disappear from certain 

 areas, does not break up into a 

 series of definite bands. 



The monotrochse or cephalo- 

 trochae are larvas in which only a 

 single pra;-oral ring is developed 

 (fig. 150 B). 



In the telotrochae there is 

 present a pros-oral and a post-oral,' 

 i.e. peri-anal ring (fig. 150 A) ; the 

 latter sometimes having the form 

 of a peri-anal patch. 



The polytrochae are segmented 

 larvae with perfect or imperfect 



rings of cilia on the seo-ments of 



, e , , ,, . v. pras-oral ciliated baud; w. peri-anal 



the body usually one ring to each ciliated band. 



FIG. 150. Two CH.ETOPOD LARVAE. 



(From Gegenbaur.) 

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



B. E. 



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