XIII 



PHYLUM CHORDATA 



15 



or.ap 



from the ectoderm, but from the underlying mesoderm, from which 

 they migrate through the ectoderm to the outer surface. These 

 formative cells of the test are to be found scattered through its 

 substance. Running through it are also a number of branching 

 tubes lined with cells, each terminal branch ending in a little 

 bulb-like dilatation. The interior of each tube is divided into 

 two channels by a longitudinal septum, which, however, does not 

 completely divide 

 the terminal bulb. 

 Through these tubes 

 (which are of the 

 nature of looped 

 blood-vessels) blood 

 circulates, passing 

 along one channel, 



ter- 



and 



alrap 



est 



gonod. 



r/iant 



go nod 



JTLCtnt 



through the 

 minal bulb, 

 back through the 

 other channel. 



When the test is 

 divided (Fig. 725) 

 the soft wall of the 

 body or mantle 

 (mant.), as it is 

 termed, comes into 

 view ; and the body 

 is found to be freely 

 suspended within 

 the test, attached 

 firmly to the latter 

 only round the oral 

 and atrial apertures. 

 The mantle (body- 

 wall) consists of the 

 ectoderm with 

 underlying layers of 

 connective -tissue 

 enclosing muscular 

 fibres. It follows 

 the general shape 

 of the test, and 

 at the two apertures is produced into short and wide tubular 

 prolongations, which are known respectively as the oral and 

 atrial siphons (Fig. 727, or. siph., atr. siph.). These are continuous 

 at 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 



FIG. 725. 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 ; 

 yon. gonad ; gonod. gonoduct ; hyp. neural gland ; hyp. d. 

 duct of neural gland ; mant. mantle ; ne. gn. nerve-ganglion ; 

 ces. ap. aperture of resophagus ; or. ap. oral aperture ; ph. 

 pharynx ; stom. stomach ; tent, tentacles ; test, test. (After 

 Herdman.) 



