40 COMPARATIVE ANATOMY. 



sac is of considerable size, with its walls produced into a series of 

 parallel longitudinal folds, radiating from the inhalent orifice 

 to the mouth. A black bristle has been introduced into the 

 aperture leading from the branchial sac to the stomach an 

 aperture which has been regarded by some as the mouth, while 

 others maintain that the mouth is better represented by the 

 inhalent aperture. 



5.9. To shew the nervous system. 



The single nerve-ganglion is situated in the substance of the 

 mantle on one side of the inhalent aperture, between it and the 

 exhalent aperture, and is indicated in the specimen by the inser- 

 tion of a black bristle behind it. The ganglion is fusiform in 

 shape, and from its anterior and posterior ends nerves are seen 

 passing towards the inhalent and exhalent apertures respec- 

 tively. 



The genital ducts are seen emerging from the lower margin 

 of the first intestinal coil, and following the course of the intes- 

 tine open into the atrium in close contiguity to the anus. 



60. An Ascidian, of a different species, dissected to shew its 

 digestive tract. 



The external test has been removed, and the mantle and wall 

 of the branchial sac reflected. 



A black bristle has been passed through the inhalent aper- 

 ture, branchial sac, and mouth, into the stom,ach, and a red rod 

 through the exhalent aperture and atrial chamber. The 

 mouth is situated at the bottom of the branchial sac, and leads 

 by a short cesophageal canal into an oblong stomach. Proceed- 

 ing from the stomach the intestine describes two curves, the 

 first of which has its convexity directed towards the nerve- 

 ganglion (marked by a black bristle being placed under it), and 

 its concavity towards the heart. Hence the primary flexure of 

 the intestine is said to be haemal. The second curve has its 

 concavity towards the ganglion, and terminates in a rectum 

 which has its anal orifice in the atrial chamber. The floor of 

 the stomach is seen to be marked by a longitudinal ridge, which 

 is prolonged into and along the intestine, so as partially to divide 

 the tube into two canals, by which arrangement the absorptive 

 surface is much increased. On the side of the branchial one 

 opposite to the mouth is to be seen a portion of the 'endostyle.' 



