PHYLUM ANNUL AT A 



457 



ie Chaetopoda, with a ccelomic epithelium, which is sometimes 



Hated. 



The alimentary canal in the Incrmia consists of a muscular 

 pharynx, intestine, and rectum ; in the Sipunculid.ee (Fig. 354) the 

 intestine is bent on itself, and spirally twisted as it runs forwards 

 to the anal opening, which, as already noted, is situated far forwards 

 on the dorsal surface : at the junction 

 of intestine and rectum is a single simple 

 caecum or a pair ; and a number of small 

 branching caeca are connected with the 

 rectum close to the anal opening. Re- 

 tractor muscles pass from the body- wall 

 to the pharynx. In the Armata, (Figs. 

 359 and 361) there is a thin-walled 

 buccal cavity, an elongated and coiled 

 intestine, opening at the posterior ex- 

 tremity of the body into a dilated rec- 

 tum : in most there is an elongated 

 cwcum or siphon applied to the ventral 

 aspect of the intestine proper. Into 

 the rectum there open a pair of remark- 

 able caeca, the posterior nepliridia (Figs. 

 359 and 3Q\,post. neph.), supposed to be 

 excretory in function ; these open into 

 the coelome by means of a number of 

 ciliated funnels (Fig. 360). 



There are no specialised organs of 

 respiration in the Gephyrea. A blood- 

 vascular system is sometimes present, 

 sometimes absent. When present, as it 

 is in most Gephyrea, it usually com- 

 prises a contractile dorsal vessel closely 

 applied to the intestine, and a peri- 

 pharyngeal ring or plexus. Cilia are 

 present in places in the interior of the 

 vessels. 



The nervous system (Figs. 355 and 

 362) consists of a nerve ring, sometimes 

 greatly elongated, surrounding the an- 

 terior part of the alimentary canal, with sometimes a dorsal and 

 anterior thickening representing a cerebral ganglion; and of a 

 nerve-cord, devoid of ganglia, running backwards from this along 

 the middle of the ventral surface, and giving off pairs of branches 

 at regular intervals; the branches of the same pair sometimes 

 form complete rings (Fig. 362, ne. ri.) by uniting dorsally. Eyes 

 of a very simple character, consisting of mere spots of pigment, 

 are present in some of the Inerrnia. 



re,sjb 



FIG. 358. Priapulus. entire 

 animal, rcsp. posterior papillae. 

 (After Ehlers.) 



