16 



ZOOLOGY 



SECT. 



their margins with the margins of the apertures of the test, and 

 round the openings are the strong sphincter muscles by which 

 closure is effected. In the rest of the mantle the muscular fibres 

 are arranged in an irregular network, crossing one another in all 

 directions, but for the most part either longitudinal or transverse. 

 Within the body-wall is a cavity, the atrial or peribranchial cavity 

 (atr. cav.), communicating with the exterior through the atrial 



or.ap 



alrap 



tent 



gonod. 



mant 



FIG. 716. Dissection of Ascidia from the right side. The greater part of the test and 

 mantle has been removed from that side so as to bring into view the relation of these layers 

 and of the internal cavities and the course of the alimentary canal, etc. an. anus ; atr. ap. 

 atrial aperture ; end. endostyle ; gon. gonad ; gonod. gonoduct ; hyp. neural gland ; hyp. d. duct 

 of neural gland ; mant. mantle ; ne. gn. nerve-ganglion ; oes. ap. aperture of oasophagus ; or. ap. 

 oral aperture ; ph. pharynx ; stom. stomach ; tent, tentacles ; test, test. (After Herdman ) 



aperture : this is not a coelome, being formed to a great extent by 

 involution from the outer surface. 



Pharynx. The oral aperture leads by a short and wide oral 

 passage (stomodceum) into a chamber of large dimensions, the 

 pharynx or branchial chamber (Fig. 716, ph.). This is a highly 

 characteristic organ of the Urochorda. Its walls, which are thin 

 and delicate, are pierced by a number of slit-like apertures, the 



