480 



ZOOLOGY 



SECT. 



ant set 



Saccosoma, and Epithetosoma. They are unsegmented in the adult 

 condition and have no parapodia, but, except in the case of Sacco- 

 soma, have a pair of ventral setae in one or both sexes. The general 

 shape is cylindrical or ovoid, with the mouth anterior and the anus 

 posterior. Overhanging the mouth is a median appendage usually 

 termed the proboscis (absent in Saccosoma). This is sometimes of 

 great length, sometimes comparatively short, and is highly sensitive 

 and mobile, but is not retractile. In some cases it contains a 

 cavity communicating with the ccelome. There is a wide ccelome 

 undivided by mesenteries, but crossed by numerous muscular 

 strands which support the alimentary canal. The alimentary 

 canal is a greatly coiled tube with a short 

 thick-walled pharynx in front and an ovoid 

 rectum behind. With the intestine communi- 

 cates at both ends an elongated tube the 

 siphon, comparable to that of the Echinoids 

 and a ciliated groove runs throughout its length. 

 There is a closed blood-vascular system with a 

 peri-intestinal sinus, a dorsal vessel, and a 

 ventral vessel. The most important part of the 

 nervous system (Fig. 395) takes the form of an 

 unsegmented ventral nerve-cord giving off pairs 

 of nerves. In front this bifurcates, and the two 

 cords resulting from the division running for- 

 wards past the mouth, extend throughout the 

 length of the proboscis, at the anterior 

 extremity of which they unite to form a great 

 loop which shows no trace of ganglia. There 

 are no organs of special sense. Into the rectum 

 open a pair of caeca the anal vesicles or pos- 

 terior nepkridia, which are probably the chief 

 excretory organs. These have appended to 

 them a number of ciliated funnels which open 

 into them from the coelome. Further forward 

 there is a pair of nephridia, one of which may 

 be aborted, or there may be two or three pairs. 

 The sexes are distinct and there may be extreme 

 sexual dimorphism. The larva is a trochophore. 



Echiurus (Fig. 393) is approximately cylindrical, with a moderately 

 long proboscis which is excavated longitudinally on the ventral 

 surface by a ciliated channel at the posterior end of which the 

 mouth is situated. The surface is covered with small papillae 

 arranged in rings. On the ventral surface not far from the anterior 

 end is a pair of short simple pointed setae (ant. set), each lodged in 

 a setigerous sac and capable of being moved freely by means of 

 appropriate muscles. Surrounding the posterior (anal) end of the 

 body are a number of setae (post, set) arranged in one or two rings. 



^rNfiw 



t>ost.set 



Fia. 393. Echiurus, 

 entire animal, ant. set. 

 anterior setfe ; post, 

 set. posterior sete ; 

 prob. proboscis. (After 

 Greef.) 



