310 



THE OCEAN WORLD.' 



more evident when it is considered that these creatures are habitually 

 filled with water, which can be expelled by very slight pressure. 



The Ascidians are sometimes free, sometimes united to others in a 

 manner more or less intimate. Hence their division into the three 

 groups of simple, social, and composite Ascidians. 



Simple Ascidians attach themselves, each individual singly, to rocks 

 and other submarine bodies, and generally at a fixed depth. Ascidia 



Fig. 124. Ascidia microcosmus (Cuvier). 



micrososmus a Mediterranean species, represented in Fig. 124, may 

 be quoted as a type of this division of Ascidians. The name of 

 Microcosmus, or the little world, is probably given from its being in- 

 habited by quite an animated colony of algae and corals, which dwell 

 upon its surface, and give it a very peculiar, but not very attractive, ap- 

 pearance. The flavour of these molluscoids is very strong, which does 

 not, however, hinder the poorer dwellers on the sea shore from eating 

 them. The genus Phattusia is another type of the group. Phalhisia 

 grossularia is of a reddish colour, and about the size of a currant- 

 berry : it usually lodges itself in the oysters of certain localities. At 

 Ostend another species, Phallusia ampulloides, is found in prodigious 

 quantities in the oyster parks, and is parasitic on living lobsters. 



Social Ascidians comprehend living Tunicata, connected together 

 on a common prolongation by the roots, but free and unconnected in 

 all other respects. Ascidia pedunculata (Fig. 125) may be quoted 

 as an example. 



