Ascidians. 



19 



Before becoming fixed to a I'ucus or stone the sea-squirt 

 has enjoj'ed for a time a free-swimming larval existence, and was 

 even provided with aii mipaired eye. Not unlike a frog-tadpole in 

 shape, this lar\'a has a long, finned tail, the axis of which consists 

 of a notochord, a structure, mentioned a propos of the Lancelet, 



NOTOCHORD 



Gl LL-oLITS 



FIG. 17. DIAGRAM OF ASCIDIAN LARVA. 



which is characteristic of all vertebrate animals, in their 

 embryonic condition at least, and unrepresented in the in- 

 vertebrates proper (Fig. 17). In some Ascidians, however, such 

 a larva does not exist, and the embryo passes gradually into the 

 perfect form, the development being direct, without meta- 

 morphosis, while others remain permanenth' in a condition 

 similar to the larval. 



Many of the Ascidians found between tide marks on our 

 coasts belong to the sedentary, fixed tj-pes, globular or barrel- 

 shaped, sac-like, with two short funnel-shaped apertures : one 

 at the top, which may be called the mouth, the other a little 

 lower down on the side, which corresponds to the vent. They 

 have been compared to a leather bottle witl: t«'o spouts. The 

 current of water enters the upper aperture, and after passing 

 through a large branchial sac comes out through the lateral 

 aperture. Like practically all fixed animals, Ascidians are 

 hermaphrodite. We find on our coasts several species of 

 Cynthia, solitary Ascidians varying in size from that of a 

 small pea to that of a plum. Some are brilliantly coloured — 

 yellow, orange or red. Ciona intestinalis is of verj^ elongate 

 shape, and the largest of our Ascidians, occasionally attaining 

 a length of nearly a foot. Ascidia {PhaUusia) virginea is 

 shorter, but also of a large size, measuring up to three inches 

 in length, with tough outer covering, yellowish or pinkish 

 white ; althoupli distinct, individuals sometimes form bunches. 



