188 INVERTEBRATE ANIMALS. 
CLAss II. TUNICATA.—The members of this class are not 
uncommonly called Ascidian Molluscs (Gr. askos, a wine-skin), 
from the resemblance which many of them exhibit in shape to 
a two-necked leather bottle (fig. 97, 2). They are popularly 
known as “Sea-squirts,” from their power of forcibly ejecting 
water from the orifices of the bottle. Their scientific name, 
again, of Zunzcata, 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 con- 
siderable proportion of a substance apparently identical with 
Fig. 97.—Morphology of Tunicata. 1. Diagram of a'Tunicary (after Allman); @ 
Oral aperture ; 4 Atrial aperture ; c Respiratory sac, with its rows of ciliated 
apertures ; d Alimentary canal ; e Anus ; /Cloaca or atrium ; ¢ Nervous ganglion. 
2. A simple Ascidian (Cynthia papillosa.) 
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. 97), one 
is perforated by the aperture of the mouth, whilst 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 
