492 



ZOOLOGY 



SECT. 



series of tooth-like processes occur round the anus, and in front a circlet of 

 adhesive papillae. In Protodrilus there is a ventral ciliated groove. There 

 is a vascular system with dorsal and ventral longitudinal vessels. In each 

 segment is a pair of simple nephridia. In Protodrilus there are two ventral 

 nerve-cords, connected together by transverse commissures : in Polygordius 

 the cord (Fig. 408, V. Nv. Cd) is single ; in neither genus is there any trace of 

 ganglia. The sexes are united in most individuals of Protodrilus, ovaries 

 occurring in all the first seven segments and testes in some of those immediately 



following. In Polygordius the sexes are 

 separate; the ovaries or testes (Fig. 408, 

 Spy) are developed in the posterior seg- 

 ments. There are no special reproductive 

 ducts. 



FIG. 407. Protodrilus, en- 

 tire animal, int. intestine ; 

 mus. oe. muscular append- 

 age of oesophagus ; oss. ceso- 

 phagus. (After Hatschek.) 



Cd 



FIG. 408. Polygordius neapolitanus, transverse 

 section of a male specimen. Cod. Epthm. parietal layer 

 of ccelomic epithelium ; God. Epthm.' visceral or 

 splanchnic layer of the same ; Cu. cuticle ; Der. 

 Epthm. deric epithelium ; D. V. dorsal vessel ; Ent. 

 Epthm. enteric epithelium ; M. PI. muscle-plates ; 

 0. M. oblique muscles ; Spy t immature gonads : 

 V. Nv. Cd. ventral nerve cord continuous with deric 

 epithelium ; V. V. ventral vessel. (From Parker's 

 Biology," after Fraipont.) 



The larva of Polygordius is a typical trochophore (Fig. 409), and its 

 metamorphosis into the adult worm (Fig. 410) takes place as in the Polychseta 

 in all essential respects. 



Ctenodrilus and its ally Zeppdinia resemble Polygordius in the ectodermal 

 position of the nervous system and in the presence of ciliated pits ; but they 

 have a number of setae on each segment, and only a single pair of nephridia, 

 which are situated in the head. There is only a single (longitudinal) layer of 

 muscles in the body-wall. Ctenodrilus multiplies asexually by fission, and 

 is hermaphrodite and viviparous. Also sometimes supposed to be related 

 to the Archi-annelids is Nerilla, a minute marine worm with parapodia and 

 setae and with hollow cirri. [Compare Dinophilea and Histriobdettea, 

 Section VII.] 



