498 



ZOOLOGY 



sometimes with oblique and internal circular layers superadded. 

 There is an extensive undivided ccelome, lined, as in the case of 

 the Chaetopoda, with a coelomic epithelium, which is sometimes 

 ciliated. 



The alimentary canal in the Incrmia consists of a muscular 

 pharynx, intestine, and rectum; in the Sipuncididce(Fig. 388) 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 coecum or a 

 pair of cseca ; and a number of small 

 branching glandular appendages are at- 

 tached to the rectum close to the anal 

 opening. Retractor muscles pass from 

 the body-wall to the pharynx. In the 

 Armata (Figs. 393 and 395) there is a 

 thin-walled buccal cavity, and an elong- 

 ated and coiled intestine, opening at 

 the posterior extremity of the body into 

 a dilated rectum : in most there is an 

 elongated caecum or siphon applied to the 

 ventral aspect of the intestine proper. 

 Into the rectum there open a pair of re- 

 markable caeca, the posterior nephridia 

 (Figs. 393 and 395, post, neph.), supposed 

 to be excretory in function ; these open 

 into the ccelome by means of a number 

 of ciliated funnels (Fig. 394). 



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. 389 and 396) consists of a nerve- 

 ring, sometimes greatly elongated, surrounding the. anterior part of 

 the alimentary canal, with sometimes a dorsal and anterior thick- 

 ening 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 inter- 

 vals ; the branches of the same pair sometimes form complete 

 rings (Fig. 396, ne. ri.) by uniting dorsally. Eyes of a very 



Fig. 302— Priapulus, entire 

 animal, resp. posterior papilte. 

 (After Bhlers.) 



