388 



COLLEGE ZOOLOGY 



organ consisting of a hollow tube of cells; it opens posteriorly 

 into the alimentary canal. The alimentary canal is straight. 

 Mud in which the animals live is taken into the mouth (10) and 

 forced slowly through the digestive tube, where nutriment is ex- 

 tracted from the organic matter contained in it — a process 

 similar to digestion in the earthworm (p. 219). The gill-slits 

 or branchial clefts (16) open into the anterior portion of the ali- 

 mentary canal and supply water to the 

 tongue-like respiratory organs. 



There is a dorsal blood-vessel (12) ending 

 anteriorly in a contractile heart (7) which 

 lies in a pericardial cavity (6). A ventral 

 blood-vessel (15) is connected with the 

 dorsal blood-vessel in the collar region by 

 two lateral tubes. The other blood- 

 vessels are simply spaces in the tissues. 

 Excretory products appear to be ex- 

 tracted from the blood by the glomerulus 

 or kidney (5), which lies on the posterior 

 wall of a cavity in the proboscis (4) . The 

 excretions pass out through the proboscis 

 pore (8) when water is expelled from the 

 proboscis cavity. 



The nervous system is not concentrated. 

 A layer of nerve- fibers just beneath the ectoderm makes the 

 entire surface sensitive. Thickenings occur along the mid-dorsal 

 and mid- ventral lines of the trunk and around the trunk just 

 posterior to the collar. A neural tube (17) is formed by the dorsal 

 thickening. The ccelom which arises from the primitive digestive 

 tract, very much as in echinoderms (p. 210, Fig. 150, A, cce), is 

 represented by a proboscis cavity (Fig. i>2)3i 4)> two collar 

 cavities (p), and two trunk cavities. 



The sexes are separate. The ovaries or testes form a double 

 row in the anterior trunk region, and the germ-cells reach the 

 exterior through pores in the body-wall. In some species each 



Fig. 334. — Tornaria 

 larva of Enteropxeusta. 

 A, anus; 0, mouth; 

 S, apical plate; W, rudi- 

 ment of proboscis ccelom. 

 (From Sedgwick's Zool- 

 ogy, after Metchnikoff.) 



