GENERAL ZOOLOGY 



A sac-like digestive gland, called the liver, is attached by a duct to the 

 anterior end of the intestine. The pharynx is surrounded laterally and 

 ventrally by a cavity, the atrium, which is enclosed by flaps of the body wall 

 fused along the midventral line. The atrium communicates with the exterior 

 through the atriopore. In the normal activities of the animal, sea water is 

 drawn into the mouth and pharynx through the space enclosed by the hood 

 and cirri. From the pharynx the water passes through the gill slits into the 

 atrium and outward through the atriopore. Microscopic food particles are 

 strained from the water in the pharynx and passed to the intestine, as in 

 tunicates. The action of ciliated cells lining the gill slits and other parts 

 is principally responsible for maintaining the flow of water that brings in 

 food materials and serves for gaseous exchange. 



The circulatory system includes a ventral blood vessel, extending the length 

 of the digestive tract, and a dorsal vessel, which is divided into right and 

 left branches in the region of the pharynx. The dorsal and ventral vessels 

 are interconnected, anteriorly by vessels in the walls of the pharynx and 

 posteriorly by a network of vessels in the digestive tract and body wall. There 

 is no heart, the circulation of the blood being brought about by peristaltic 

 contractions of the longitudinal vessels. The course of the blood, anteriorly 

 in the ventral vessel, upward in the ves.sels of the gill slits, and posteriorly 

 in the dorsal vessel, resembles the course of circulation in vertebrate embryos 

 and in fishes. 



There is a conspicuous coelom in the region of the intestine, but anteriorly 

 the coelom is reduced in the region of the pharynx. Numerous excretory 

 organs, in the form of modified nephridia, open into the atrium; they occur in 

 bilateral pairs and are segmentally arranged. It is a peculiar fact that so- 

 lenocytic protonephridia also occur in the amphioxus. As seen in the in- 

 testinal region, the lateral parts of the body are largely occupied by the 

 somatic musculature; anteriorly, the muscles fill only the dorsolateral 

 quadrants. The notochord lies dorsal to the digestive tract, surrounded by 

 the musculature; it is thus in a position completely comparable with that of 

 the notochord in vertebrate embryos. The connective-tissue sheath surrouncl- 

 ing the notochord extends dorsally as a sheath for the central nervous system, 

 which is an axial, tubular cord resembling the spinal cord of a vertebrate. 

 The cavity of this tube is slightly expanded at the anterior end, where there 

 are a supposed olfactory organ and a median pigment spot. There is ho 

 anterior difTerentiation of the nerve cord that can be fairly called a brain, 

 although two pairs of nerves extending anteriorly have been termed cranial 

 nerves. Posterior to these, a large number of bilaterally paired nerves pass 

 from the nerve cord in a metameric arrangement corresponding with that of 

 the muscle segments. The cord contains photoreceptors in the form of pig- 

 mented, cup-like bodies. 



The reproductive organs consist of paired gonads, segmentally disposed and 

 lying in the lateral walls of the atrium, into which they project. The sexes 

 are separate, and the gametes are discharged into the atrium by rupture of the 



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