MOLLUSCA: TUNICATA. 



381 



of these being an ingestive and branchial aperture, whilst the 

 other serves as an excretory aperture. The covering of an 

 Ascidian is composed of two layers. Of these the outer is 



br 



Fig. 200. Morphology of Tunicata. A, Diagram of the structure of a simple Tuni- 

 cate : / Test ; t' Second muscular tunic ; j Branchial sac ; b Branchial aperture ; a 

 Atrial aperture ; c Atrium ; o Opening of the gullet ; g Stomach, leading into the 

 intestine; an Anal aperture ; n Nerve-ganglion. B, Botryllus smaragdus a small 

 portion of a colony of the natural size, and a single system of the same enlarged; co 

 Common atrial aperture ; br Branchial aperture of one of the zooids. C, Molgula 

 Manhattensis, a simple Ascidian. The arrows in A and C show the direction of the 

 water-currents. 



called the " external tunic," or " test," and is distinguished by 

 its generally coriaceous or cartilaginous consistence. It is also 

 remarkable for containing a substance which gives the same 

 chemical reactions as cellulose, and is probably identical 

 with this characteristic vegetable product. Sometimes it con- 

 tains spicules or plates of calcareous matter. The test is lined 

 by a second coat, which is termed the "second tunic," or 

 " mantle," and which is mainly composed of longitudinal and 

 circular muscular fibres. By means of these the animal is en- 

 dowed with great contractility, and has the power of ejecting 

 water from its branchial aperture with considerable force. 

 The mantle lines the test, but is only slightly and loosely at- 

 tached to it, especially near the apertures. The ingestive or 

 branchial aperture (fig. 200, A, b) is generally surrounded by a 

 circlet of small, non-ciliated, non-retractile tentacles, and opens 

 into a large chamber (fig. 200, A, s), which usually occupies 



