CHORDATA 



215 



called myocommas. The dorsal body wall is greatly thickened as 

 it is in all Chordates and contains the hollow nervous system and 

 the notochord. The notochordal sheath of connective tissue is 

 produced dorsally into a covering for the canal containing the nerv- 

 ous system. The cells are 

 turgid with fluid. 



Digestive System . — The 

 mouth has ciliated oral cirri, 

 with tactile sensory cells. 

 The twelve velar tentacles 

 extend as strainers from the 

 velum across the mouth. 

 The pharynx with its one 

 hundred pairs of gill slits 

 functions in respiration as 

 well as digestion. The epi- 

 branchial groove is ciliated, 

 dorsally indenting the phar- 

 ynx, and the ventrally situ- 

 ated endostyle (Figure 102) 

 (hypobranchial groove) fur- 

 nishes mucus which entangles 

 food particles. The food is 



carried by the cilia of the „ _ .• ^ ^i- ,, 



•^ . Fig. 102. Cross-section Amphioxus. 



epibranchial groove to the m- Hertwig-Kingsley. a, aorta descendens; b, 

 testine. The liver, attached peribranchial space; c, notochord; ro, coelom 

 to the anterior end of the (branchial body cavity); e^ hypobranchial 

 intestine, protrudes into the groove, beneath It the aorta ascendens; g, 

 1 1 ;,.,, TU^ V,o gonad; ^^, gill arches; ^i, pharynx; /, liver; »7, 



pharyngeal cavity. Ine he- & ' '^ ' '^ ^ 



^ ■' ° . •J- • muscles; «, nephndia, on the lert with an 



patlC secretion is a digestive ^..^w; r, spinal cord; .«, spinal nerve; .;>, gill 

 juice probably analogous to glit. (After Lankester and Boveri. Courtesy 

 the secretion of the pancreas of Henry Holt & Co.) 

 of the vertebrates. 



Respiratory System. — The atrium is a wide chamber between the 

 body wall and the pharynx into which the gill clefts lead. As in the 

 tunicates the cilia lining the gill clefts produce a current setting in 

 at the mouth, entering the pharynx and passing thence by gill slits 

 into the atrium and out the atriopore. The current is for respiration 

 as well as food. The coelom consists of paired cavities in the pharyn- 

 geal region connected by narrow canals in the gill folds with the 

 endostyle. 



