2 46 CUTTLE-FISH, MOLLUSC A, ETC. 



in size, but all have been or are kept in captivity at 

 the Crystal Palace. At Brighton, some of them, as 

 well as species of echmoderms, appear to be kept with 

 great difficulty, or cannot be preserved at all. 



Fig. 178. 



Donax politus. 



Around the margins of the rockwork in most of the 

 large marine tanks may be seen rows of greenish- 

 white objects, not unlike elongated white Hamboro' 

 grapes, which are fixed by their bases. These are the 

 Ascidia, a very interesting group of mollusca allied 

 to the polyzoa, which some naturalists have actually 

 included among the Vertebrate animals. For reasons 

 which are partly structural and embryonic, they are 

 usually regarded, however, as nearly related to the 

 mollusca. They are in reality soft-coated mollusca, 

 just as oysters and others are hard- coated. Four 

 species of these ascidians, Ascidia intestinalis, A. 

 vitrea, Molgula tubulosa, and Cynthia quadrangularis, 

 have semi-spontaneously made their appearance in 



