604 HISTORY OF SYSTEMS. 



Normal Group ? M. Bifouidjj. 



Thalida. 



their young 

 state, and float- 

 ino' SaIjE'. 



Or such Tunica- 

 ta as have their 

 mantle adhering 

 everywhere to the 

 envelope ; their j 

 branchife irregular, ' 

 consisting of two fo- 

 liated processes at- 

 tached to the sides 

 of the thorax; and 

 their branchial ori- 

 fice merely provi- 

 ded with a valvule. V5- ***** ***** 



Milne-Edwards, in a work * inferior to none of the many 

 excellent ones with which he has enriched zoology, has pro- 

 posed a new tribe or order in the class constituted of some 

 recently discovered genera, — named Clavelina and Pero- 

 phora ; — because each species consists of several individuals 

 that pullulate from a common creeping tu:bular root, by 

 which they are associated together organically, and so in- 

 timately that there is even a circulation established between 

 the different individuals of the same group. This section 

 Milne-Edwards calls the social, — AsciDiES sociales, — and it 

 has been admitted by Professor Forbes. I think that the claim 

 to this distinction cannot be allowed. The character on 

 which it is asserted is one of secondary importance, as seems 

 to me proved by the fact that the gap which would separate 

 them from the simple Ascidians is greatly narrower, and less 

 evident, than that which wovild separate the social from the 

 compound Ascidians. In habit and structure, the social do 

 not differ in any way from the simple kinds, but the so- 

 cial do differ very much in both particulars from the 

 compound kinds. Hence, I think that they ought to be 

 reckoned merely as a family in the tribe of Ascidiadae. 



Professors Forbes and Goodsir have proposed (1841) the 

 establishment of another tribe under the name of Pelonia- 

 D^, with a better reason, for there is a difference in habit 

 and economy which justifies the conclusion that would be 

 drawn from the anatomical structure. The Peloniadas are 

 simple Tunicata, but they are free, and have a habit that 

 might seduce us to look for their relations among the worm- 



* Observations sur les Ascidies composees des Cotes de la Manchc. 

 Paris, 1844. 4to. 



