PHYLUM ANNULATA 



451 



septa. The ventral nerve-cord is not made up of a series of 

 ganglia. There is, as a general rule, only a single pair of nephri- 

 dia. The sexes are separate ; the ovaries and testes simple masses 

 of cells ; the iiephridia act as reproductive ducts. The larva is 

 a trochosphere. 



1. EXAMPLE OF THE CLASS Sipunculus nudn. 



-post. pap 



General External Features. Siimncuhis occurs on sand at 

 moderate depths off the coast in most countries outside of the 

 tropics. It is an elongated worm of a cylindrical shape, somewhat 

 narrower towards one the anterior end. There is no trace of 

 division into segments. The anterior portion of the body, to the 

 extent of about a sixth of the total length, is capable of being 

 involuted within the part be- 

 hind. The surface of this an- 

 terior part, which is termed the 

 introvert (Fig. 352), differs in 

 appearance from that of the 

 rest of the body in being 

 covered more or less closely 

 with chitinous papillae. The 

 papilla? of the posterior portion 

 of the introvert are shaped 

 like the bowl of a spoon, with 

 the concavity turned towards 



i/ 



the body-wall and the tip 



directed backwards ; they are 



so closely arranged as to overlap 



one another like the shingles 



of the roof of a house : further 



back they become longer and 



narrower, mammilliform, and 



more scattered. When the in- 



trovert is fully evagiiiated, 



there appears at its extremity a horseshoe-shaped fold of the 



integument, the tentacular fold (tent.), which is lobed and plaited 



(Fig. 353) so that it assumes somewhat the appearance of a circlet 



of tentacles. For a little space immediately behind the tenta- 



cular fold the surface of the introvert is free from papilla?. 



The posterior portion of the body is devoid of papilla-, but is 

 marked out by a number of narrow impressed lines into a number 

 of elongated four-sided areas. 



Body-wall. The surface is covered by a chitinoid cuticle 

 having an iridescent lustre similar to that presented by the cuticle 

 of Nereis and Lumbricus, and due to the same cause viz., the 

 presence of two systems of intercrossing lines. The papilla? on 



G G 2 



Fio. 352. Anterior extremity of Sipun- 

 culus nudus. ant. pup. anterior papil- 

 lary region ; /ifixi. /</<. posterior papillary 

 region; tent, tentacular fold. (After Ward.) 





