172 



INVERTEBRATE ANIMALS. 



of the bottle. Their scientific name, again, of Tunicata, is 

 derived from the fact that the body is enveloped in a leathery 

 elastic integument, which consists of different layers, and which 

 takes the place of a shell. The outer covering of the animal 

 is of a gristly or leathery consistence, and is known as the 

 " test." It is remarkable for containing a considerable propor- 

 tion of a substance apparently identical with cellulose, which 

 is one of the most characteristic of all vegetable products. 

 The test is lined by a second coat, which is highly muscular, 

 and confers upon the animal its power of contracting itself 

 and squirting out water. Of the two necks which are placed 

 at the anterior end of a simple Ascidian (Fig. 78), one is per- 



x</ 



FIG. 78. Morphology of Tunicata. 1. Diagram of a Tunicary (after Allman): a Oral aper- 



res; d Ali- 

 A simple 



ture; b Atrial aperture; c Respiratory sac, with its rows of ciliated apertures; d Ali- 

 mentary canal; e Anus; / Cloaca or atrium; g Nervous ganglion. 



Ascidian (6 



fo rated by the aperture of the mouth, while the other serves 

 as an excretory aperture. These two apertures are known 

 respectively as the " oral " and " atrial " apertures. 



The oral aperture (a) is usually furnished with a circle of 

 small non-retractile tentacles, and opens into a great chamber 

 known by various names, but best as the " respiratory sac." 

 This sac occupies the greater part of the cavity of the body 

 (Fig. 78, 1, c), and has its walls perforated by numerous aper- 

 tures, the sides of which are ciliated. At the bottom of the 

 respiratory sac is a second opening (the mouth of some 



