AND THE LOWER ANIMALS. 143 



it were into a common mass, and, having the closest 

 possible organic relation to each other, are therefore 

 styled eompowmd ascidians. The first appear as ir- 

 regularly globular masses attached to sub-marine 

 bodies ; the second are usually found suspended from 

 the roof of some rocky cavern, and have the appear- 

 ance of beautiful crystal ornaments ; the third are 

 seen covering whole rocks, stones, and sea-weeds. 

 The mass is usually of a gelatinous character, semi- 

 transparent, and tinted with red, green, or brown; 

 and upon the surface of each colony a number of 

 irregular festoon-like markings may be observed, 

 figures which are by no means unlike those traced by 

 a mathematician/ s compass. Moreover, the organiza- 

 tion of all ascidians is in the main identical. They 

 have all a very simple nervous system, a very imper- 

 fect circulatory apparatus, a more or less contorted 

 digestive canal, and in many of them, owing to the 

 extreme transparency of the integument, the whole 

 anatomy may be studied with the aid of a simple lens, 

 without having any recourse to the disaecting-knife. 



The egg of the compound ascidian is very rapidly 

 developed, and exhibits the same phenomena as those 

 described in the earlier chapters of this volume. It 

 is entirely converted into a larva like that of Hermella 

 and Teredo. This little creature is of an oval form, 

 and is furnished with a long tail which gives it some 

 resemblance to a tadpole. As yet there are no distinct 

 internal organs, but in their place is found a central 

 homogeneous yellow mass, from which they will be 

 formed, and which is surrounded by the thick, color- 

 less, and transparent envelope. A prolongation of 

 this mass extends into the tail, and three others act 



