THE PHYLUM CHORDATA 



51 



af 



endostyle, around the mouth peripharyngeal grooves, 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 Amphioxus, but the machine seems to be of a much improved 

 type, appropriate for 

 purely sedentary life. 

 An atrial cavity sur- 

 rounds the pharynx, 

 which is inclosed by a 

 mantle that surrounds 

 the whole body. The 

 ectoderm of this mantle 

 it is that secretes the dl 

 tunic. The atriopore, 

 instead of being post- 

 erior in position and 

 backwardly directed, is 

 close to the mouth and 

 f orwardly directed. 

 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 

 notochord, 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 between the two funnels. It is nothing but a ganglion 

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

 ynx. 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. 



FIG. 23. Internal anatomy of a typical Asci- 

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

 pore; an, anus; dl, dorsal lamina; e, endostyle; 

 g, 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; st. stigmata or subdivided pharyngeal 

 clefts; v, velum. (Considerably modified after 

 Hertwig.) 



