CLASS TUNICATA. 5 1 



CHAPTER XIV. 



CLASS TUNICATA. 



The animals known as Tunicata are all inhabitants of the 

 sea, and derive their name from the fact that the body is 

 enclosed in a sort of bag or " tunic." They are also often 

 caUed Ascidians (Greek, askos, a wine-skin), from the fact 

 that many of them have a close resemblance to a leathern 

 wine-skin or bottle. " Rarely," remarks Prof. Edward 

 Forbes, "is the dredge drawn up from any sea-bed at all 

 prolific in submarine creatures without containing few or 

 many irregularly-shaped leathery bodies, fixed to sea-weed, 

 rock, or shell, by one extremity or by one side, free at the 

 other, and presenting two more or less prominent orifices, 

 from which, on the slightest pressure, the sea-water is 

 ejected with great force. On the sea-shore, when the 

 tide is out, we find similar bodies attached to the under 

 surface of rough stones. They are variously, often 

 splendidly, coloured; bvit otherwise are unattractive, or 

 even repulsive, in aspect. These creatures are Ascidice, 

 properly so called." As the type of this group we may 

 take Ascidia (Phallusia) mentula, a not very rare inhabi- 

 tant of British seas ; but for various reasons it wUl be 

 sufficient to indicate very briefly the chief points in its 

 anatomy. 



Ascidia nienhda (fig. 21, A) presents itself in the form 

 of an oval or oblong, leathery body, which is attached by 

 the greater portion of one side to a stone or to a shell, 

 and which has one extremity drawn out into two promi- 

 nent necks, each perforated by a distinct opening. The 

 animal thus has very much the appearance of a two- 

 necked jar. If allowed to rest in a basin of sea- water, it 

 can be seen that a current of water is drawn in by the 

 longest or liighest of the two necks of the sac or jar, and 

 is expelled again from the lower neck. If the animal be 



