492 



/OOLOGY 



SEf'T. 



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

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

 sive coelome 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 testes simple masses 

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

 a trochophore. 



1. EXAMPLE OF THE CLASS Sipunculus nudus. 



General External Features. Sipunculus 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 aboufc 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 papillse. The papilla? 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 

 introvert is fully evaginated, 



* / m T, --,- there appears at its extremity 



W/^^lf^^/'^/'^ a horseshoe-shaped fold of the 

 f'toVjJ'jiw y'*iW.U:*;& ' integument, the tentacular fold 



(lent.), which is lobed and 



WMm. P laited ( F; g- 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 of the body is devoid of papilla?, but is marked out by a 

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

 sided areas. 



tent 



ant 



FI 



3SC>. Anterior extremity of Sipun- 

 culus nudus. ni't. imp. anterior pupil- 

 lary region ; j>o*f. /</>. posterior papillary 

 region ; tent, teutacului fold. (After Waul.) 



