492 



ZOOLOGY 



the mouth situated at the base. The anus is sometimes terminal 

 and posterior, sometimes anterior and dorsal. There is an exten- 

 sive ccelome filled with a corpusculated fluid, and not divided by 

 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 teste's simple masses 

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

 a trochophore. 



1. Example of the Class — Sipun cuius nudus. 



General External Features. — Bipunctdus 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 behind. The surface of this anterior 

 part, which is termed the introvert (Fig. 386), differs in appearance 

 from that of the rest of the body in being covered more or less 

 closely with chitinous papillae. The papillae of the posterior 

 portion of the introvert are shaped like the bowl of a spoon, 



with the concavity turned to- 

 wards 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 



i*;.t*.'t;«~.' / ,/ i.-J'th introvert is fully evaginated, 



there appears at its extremity 

 a horseshoe-shaped fold of the 

 integument, the tentacular fold 

 {lent.), which is lobed and 

 plaited (Fig. 387) so as to as- 

 sume somewhat the appearance 

 of a circlet of tentacles. For 

 a little space immediately be- 

 hind the tentacular fold the 

 surface of the introvert is free 

 from papilla?. The posterior 

 portion 6f the body is devoid of papillae, but is marked out by a 

 series of narrow impressed lines into a number of elongated four- 

 sided areas. 



post pap 



Pig. 38(5. — Anterior extremity of Sipun- 

 culus nudus. ant. pap. anterior papil- 

 lary region ; post. pap. posterior papillary 

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



