XIII 



PHYLUM CHORDATA 



13 



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

 completely divide the terminal bulb. Through these tubes (which 

 are of the nature of blood-vessels) blood circulates, passing along 

 one channel, through the terminal bulb, and back through the 

 other channel. 



When the test is divided (Fig. 674) the soft wall of the body or 

 mantle (mant.\ as it is termed, comes into view ; and the body is. 



or.ap 



at rap 



mant 



FIG. 674. Dissection of Ascidia from the right-hand side. The greater part of the test and' 

 mantle has been removed from that side so as to bring into view the relations 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. hypophysis ; hyp. d. duct 

 of hypophysis ; mftnt. mantle ; ne. cm. nerve-ganglion ; ces. ap. aperture of resophagus ; or. ap. 

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



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. tilQ,or.siph. atr. siph.). These are continuous at,- 



