12 



ZOOLOGY 



SECT. 



1. EXAMPLE OF THE CLASS THE ASCIDIAN OR SEA-SQUIRT 



(Ascidia). 



Sea-squirts are familiar objects on rocky sea-shores, where they 

 occur, often in large associations, adhering firmly to the surface of 

 the rock. When touched the Ascidian ejects with considerable 

 force two fine jets of sea-water, which are found to proceed from 

 two apertures on its upper end. The shape of the Ascidian, 

 however, can only be profitably studied in the case of specimens 

 that are completely immersed in the sea-water, specimens not 

 so immersed always undergoing contraction. 

 In an uncontracted specimen (Fig. 673), the 

 general shape is that of a short cylinder with 

 a broad base by which it is fixed to the rock. 

 The free end presents a large rounded aper- 

 ture, and some little distance from it on one 

 side is a second of similar character. The 

 former aperture is termed the oral, the latter 

 the atrial. A strong current of water will be 

 noticed, by watching the movements of float- 

 ing particles, to be flowing steadily in at the 

 former and out of the latter. When the ani- 

 mal is removed from the water both apertures 

 become narrowed, so as to be almost com- 

 pletely closed, by the contraction of sphincters 

 of muscular fibres which surround them. At 

 the same time the walls of the body contract, 

 streams of water are forced out through the 

 apertures, and the bulk becomes considerably 

 reduced. 



Body-wall and Atrial Cavity. The outer 

 layer of the body- wall is composed of a tough 

 translucent substance forming a thick test 

 or tunic (Fig. 674, test). This proves when analysed to consist 

 largely of the substance cellulose, which has already been referred to 

 (vol. i. p. 14) as a characteristic component of the tissues of plants, 

 and which is rare in its occurrence in the animal kingdom. The 

 test of an Ascidian is frequently referred to as a cuticle, and it is 

 a cuticle in the sense that it lies outside the ectoderm. The cells 

 which form it, however, seem to be chiefly derived, not from the 

 ectoderm, but from the underlying mesoderm, from which they 

 migrate through the ectoderm to the outer surface. These for- 

 mative cells of the test are to be found scattered through its 

 substance. Running through it also are a number of branching 

 tubes lined with cells, each terminal branch ending in a little 

 bulb-like dilatation. The interior of each tube is divided into 



FIG. 673. Ascidia, 

 entire animal seen 

 from the right-hand 

 side. (After Herd- 

 man.) 



