238 INVERTEBRATE M0RPH0L0G7. 



only by a pair of setae situated on the ventral surface of the 

 body, nearer the anterior than the posterior end. The body- 

 wall presents a close similarity in its structure to that of the 

 Chaetopods — differing, however, in the occurrence of a more or 

 less pronounced layer of fibres having an oblique direction. 

 The ccelom is lined by a layer of flat peritoneal cells, but 

 shows no division into more or less distinct compartments, no 

 trace of metamerism, but, as in the Chsetopods, the peritoneal 

 lining is reflected upon the walls of the digestive tract, form- 

 ing mesenteries suspending the intestine. As a rule the dor- 

 sal mesentery disappears, and in some cases the ventral one 

 is almost wanting, the intestine being slung only by a 

 number of irregular strands of connective tissue extending 

 from it to the body-wall. In some forms {Sipunculus) the sur- 

 face of the peritoneum, especially that covering the intestine, 

 is dotted with numerous irregularly scattered minute depres- 

 sions, whose openings are guarded each by a peculiar ciliated 

 ceil, and which contain cells comparable in function to the 

 chloragogue cells of the Chsetopoda. The coelomic cavity is 

 occupied by a haemolymph, which in some cases is colored, 

 and contains numerous cell-elements, some of which may be 

 circular in outline and colored by haemoglobin, while others 

 are amoeboid and colorless. 



A blood vascular system, principally developed in the an- 

 terior portion of the body, is present and appears to be com- 

 pletely closed, though connections with the ccelom are said to 

 exist in some forms. In Sipimcidus, for instance, the system 

 consists of a collar surrounding the oesophagus, sending 

 branches into the tentacles which surround the mouth, and 

 dorsally dilating into a wide sinus lying just below the brain ; 

 and from this sinus a dorsal vessel (Fig. 109, Bs) passes 

 backwards along the digestive tract for a short distance, end- 

 ing blindly where the oesophagus joins the stomach. In EcMu- 

 rus a ventral vessel runs the entire length of the body just 

 above the nerve-cord, and it is united with the dorsal vessel 

 by lateral vessels at its anterior and posterior extremities. 



The digestive tract may be either straight (Priapulvs) or 

 considerably convoluted {EcMurus and Sipunculus, Pig. 109, 

 Jnt), and the anus is in some forms terminal {Echiurus), while 



