14 



ZOOLOGY 



SECT. 



Chordata ; its affinities with that phylum are only detected when 

 the life-history is followed out, the notochord and other higher 

 structures becoming lost in the later stages of the metamorphosis. 

 Multiplication by budding, so common in the lower groups of 

 Invertebrata, but exceptional or absent in the higher, is of very 

 general occurrence in the Urochorda. 



1. EXAMPLE OP 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. 

 724), 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 

 aperture, 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 floating particles, 

 to be flowing steadily in at the former and out 

 of the latter. When the animal is removed from 

 the water both apertures become narrowed, so 

 as to be almost completely closed, by the con- 

 traction of sphincters of muscular fibres which 

 surround them. At the same time the walls of 

 the body contract, streams of water are forced through the aper- 

 tures, and the bulk is 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. 725, test). This proves when analysed 

 to consist largely of a substance called tunicine, which is apparently 

 identical with the cellulose already referred to (Vol. I., p. 15) as a 

 characteristic component of the tissues of plants, and of rare 

 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 and is derived from that layer 

 in the first instance. The cells, however, by the action of which its 

 substance is added to in later stages, seem to be chiefly derived, not 



FIG. 724. Ascidia, 



entire animal seen 

 from the right side. 

 (After Herdman.) 



