M. Van Beneden on the Physiology of the Simple Ascidians. 258 
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 Aseidia. 
In my opinion we ought to return to the classification of Lin- 
neeus 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 Linneus 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 Medusa, the An- 
thozoa and the Alcyonians. 
In the class Vermes there remain the Nematoidea, the Acan- 
thotheca, &e. In the Echmodermata 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. 
Lastly, 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 ; 
