OF THE FIXED AND FREE TUNICATA. 173 



forest, Nerites breathing in the mountain streams, Nerites in brackish waters, 

 Nerites on the beach in fellowship with Littorina, and in the sea itself, one would 

 say that habit of life would afford anything but a natural character. Nor indeed 

 would it in the case given, and in many others that might be adduced ; but it 

 certainly affords us the simplest and most natural primary division of Tunicata^ 

 and the other characters adopted in the table appear to answer their purpose 

 equally well. 



The idea has been too commonly entertained, that the Pelagic Tunicata, so 

 called, compose a group only commensurate with the compound, the social, or the 

 simple, taken separately, but I trust that a candid analysis of the preceding table 

 will serve to free the judgment from an accustomed bias in this particular, and 

 show that the fixed and the free Tunicata form two subclasses of at least nearly 

 equal value in a zoological point of view ; and I almost imagine, though myself 

 affected by the prejudice to which I have alluded, that the balance is rather in 

 favour of the Pelagians. 



Though we may readily, as by a kind of empiricism, recognise a difference 

 between the simple^ social, and compound Tunicata, it is by no means easy to give 

 them an intelligible definition. Indeed, I can safely say, as a student of the Tuni- 

 cata, that I have long known more of the existence of a difference than of its pre- 

 cise nature, and this can scarcely be arrived at by the study of books alone. In- 

 deed, the whole subject is even now shadowy and ill defined, notwithstanding the 

 great light that has been shed upon it by the labours of Savigny, Macleay, 

 Fleming, Milne-Edwards, and, in particular, Professor Huxley. 



The term Compound is just as applicable to a tree of Perojphora, as it is to one 

 oi Sertularia ; but that term is restricted to another group, including forms that 

 differ remarkably inter se, such as Botryllus and Sigillina, for example, their 

 great characteristic being, as far as I can see, more or less complete immersion of 

 the zooids in a common test, with or without vascular intercommunication. This 

 immersion of the zooids is an important feature, as no common cloacal system 

 can otherwise exist ; but, inasmuch as it may occur without the formation of 

 cloacae, it links the social group with such compound Tunicaries as possess a com- 

 mon cloacal system. Furthermore, the branched and undermining form of this 

 system, in several genera, indicates the passage to the Botryllian punctate, linear, 

 or reticulate type, in connection with which latter, as a genetic character, several 

 zooids are developed from a single ovum, as in Pyrosoma. 



The process of gemmation, on the other hand, is much more energetic in the 

 social than in either the simple or compound Ascidians. Thus, we scarcely ever 

 find incipient buds springing from, or beyond others little farther advanced in the 

 two latter, while such is the rule in the former group. 



That the increase of the connecting substance, or " ascidiarium" of Huxley, 

 proceeds pari passu with the gemmae, and is, in fact, in advance of them in the 



