. A manual of zoology. 3'4 MANUAL OF ZOOLOGY nervous system as in Balanoglossus. They both differ from Balano- glossus in having the alimentary canal bent on itself, so that the anal opening is situated not far from the mouth, in the presence of tentacles arising from the collar; and in the comparatively small size of the proboscis. 2. THE UROCHORDA Still more unlike a vertebrate in general appearance than Balano- glossus, and yet, as the earlier stages show, indubitably to be assigned to the Chordate phylum, are the Asoidians or Sea-squirts and their allies. Sea-squirts are familiar objects o
. A manual of zoology. 3'4 MANUAL OF ZOOLOGY nervous system as in Balanoglossus. They both differ from Balano- glossus in having the alimentary canal bent on itself, so that the anal opening is situated not far from the mouth, in the presence of tentacles arising from the collar; and in the comparatively small size of the proboscis. 2. THE UROCHORDA Still more unlike a vertebrate in general appearance than Balano- glossus, and yet, as the earlier stages show, indubitably to be assigned to the Chordate phylum, are the Asoidians or Sea-squirts and their allies. Sea-squirts are familiar objects on rocky sea- shores, where they occur often in large associa- tions, adhering firmly to the surface of the rock. They also live free in sand and in mud, at differ- ent depths, some being deep-sea forms. 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 l (Fig. 197) 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 contrac-. FlG. 197. —Ascidia, en- tire animal seen from the right - band side. (After Herdman.) 1 The European species, whose anatomy is here described, is approxi- mately represented by our common large Ascidia callosa, which lives in deep water
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