THE PHYLUM CHORDATA 



51 



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WnuLilillLr' 



endostylc, arouinl the mouth peripharyngeal groores. and on the dorsal 

 side a dorsal lamina (corresponding to the dorsal groove of Amphioxus) ' 

 The method of food concentration and transportation is similar to 

 that of .\niphioxus, but the machine seems to lie of a much improved 

 type, appropriate for 

 purely sedentary life. 

 An atrial ean'ty sur- 

 rounds the ]iharynx, 

 which is inclosed by a 

 nianilc that surrounds 

 the whole body. The 

 ectodenn of this mantle 

 it is that secretes the cH 

 tunic. The atriopore, 

 instead of being post- 

 erior in position and 

 backwardly directed, is 

 close to the mouth and 

 forwardly d i r e c t e d . 

 The stomach opens near 

 the bottom of the phar- 

 ynx and the intestine 

 takes a complete turn 

 and opens forward into 

 the atrium. There is no 

 noiocJwrd, no neural tube; 

 indeed almost none of 

 the structures character- 

 istic of the dorsal side 

 are present. A poor ex- 

 cuse for a brain occurs 

 on the dorsal side of the 

 mouth embedded in the 

 body wall lietween the two fumiels. It is nothinii luit a ganglion 

 associated with a sub-neural gland and with a duct entering the phar- 

 A'ux. This last structure has been compared to the vertebrate hypo- 

 physis. The ascidian comes as near being an animated pharynx as 

 could well be, only certain absolutely essential elements of the other 

 parts of the body being retained. 



y 



Fig. 23. — Internal anatomy of a t>-pical .Asci- 

 dian. a, atrial cavity; of, atrial funnel or atrio- 

 pore; an, anus; (//, dorsal lamina; c, endostyle; 

 (?, gonad; gd, duct of gonad; h. heart; hy, hypo- 

 physeal duct; i. intestine; m, mantle, ng, neural 

 gland; oe, oesophagus; of, oral funnel or mouth; 

 ■s, stomach; s/. stigmata or subdivided pharyngeal 

 clefts: r, velum. (Considerably moiUtied after 

 Hcrtwig. ) 



