M. Van Beneden on the Physiology of the Simple Ascidians. 253 



In the last part of the memoir, consecrated exclusively to what 

 has been called the zoological portion, as if zoology was reduced 

 to the distinction of species, I have availed myself of the oppor- 

 tunity to say a word upon the general classification of animals, 

 when discussing the place proper to be assigned to the Ascidia, 



In my opinion we ought to return to the classification of Lin- 

 nseus in the distribution of the animal kingdom. In invertebrate 

 animals there are only two types, viz. insects and worms, and 

 the mollusca and radiata of Cuvier, which properly make one 

 branch only. It is not the organization which ought exclusively 

 to serve as a basis for the primary divisions, but rather the em- 

 bryogeny. There are in nature animals in which the vitellus 

 enters inwards by the belly ; others in which it enters by the 

 back ; and again, others in which it enters neither by one nor 

 the other way. The first are the vertebrated, or the Hypo-coty- 

 ledones ; the second are the articulated, or the Epi-cotyledones ; 

 and the third embrace the mollusca and radiata, or the Allo-co- 

 tyledones. 



It is very remarkable that in botany the three great divisions 

 of plants into dicotyledons, monocotyledons and acotyledons has 

 been more and more corroborated by every subsequent investi- 

 gation ; and there should be, if I do not deceive myself, in the two 

 kingdoms of organic beings, the same divisions based upon the 

 same embryogenic organs. ' 



Many almost insurmountable difficulties disappear in a great 

 measure when we admit the conjunction of the mollusca and ra- 

 diata of Cuvier in one section, as Linnaeus has done. 



The Allo-cotyledones embrace several classes, just as the ver- 

 tebrated and articulated animals do. We may arrange them in 

 the following order : Mollusca, Polypes, Vermes, Echinodermata, 

 Rhizopodes, Infusoria and Sponges. In the class Mollusca we 

 may preserve the established divisions, with the exception of the 

 Cirrhipedes, which are Epi-cotyledones. The Tunicata form a 

 distinct order of the same value as the Acephala, the Gasteropoda 

 and the Cephalopoda. 



The class Polypi embraces the Bryozoa, the Medusce, the An- 

 thozoa and the Alcyonians. 



In the class Vermes there remain the Nematoidea, the Acan- 

 thotheca, &c. In the Echinodermata there are no changes re- 

 quired excepting for some doubtful genera. The Rhizopodes, 

 embracing the greater part of the microscopic Cephalopoda, in 

 my opinion should constitute a distinct class, having, at its head, 

 the Noctiluca miliaria, which seems to be a naked Rhizopoda. 

 Xastly, the classes of Infusoria and of the Sponges. 



Some zoologists consider the Cirrhipedes as crustaceans, be- 

 cause in their early age they have all the characteristics of these ; 



