284 Dr Erichson on the Classification of Invertebrate Animals. 
there is no direct passage of the legs into parts of the mouth, 
but the mouth lies behind the first pair of feet.* 
In the Linnzan‘worms, an exact further subdivision is much 
more difficult, because the external structure offers but little for 
this purpose, and our knowledge of their external organization 
still exhibits important blanks. The Mollusca undoubtedly form 
the first class, for in them the internal structure, viz. the com- ° 
position of their organs of nutrition, presents the greatest de- 
gree of perfection. A second class would be formed by all 
those worms which, with a symmetrical structure of body, ex- 
hibit a linear type of that structure, and which, like the Mol- 
lusea, have a perfect intestinal canal, with a mouth and anus, 
therefore the Annelides, Turbellarize, the Nematoides of the 
Helminthoidez, and the Rotatoriz. Here we find, along with 
a similar type of the bodily form, likewise several approxima- 
tions to insects, such as in the Nereidz to the Myriapods, and 
in the Rotatoriz to certain Entomostraca, which indeed can- 
not escape notice. A ¢hird class are the Radiata, with a ra- 
diated type in the structure of their bodies, for the most part 
with a central digestive cavity, and also a central mouth, which 
in the free-moving species is directed downwards, but in the ad- 
herent species is turned upwards. In the last classes the ali- 
mentary canal is vessel-like, and is simply branched in two in 
the Helminthoide (with exception of the Nematoidez), branch- 
ed in a tree-like manner in the Planaria, and leads to a mul- 
titude of simple stomachs in the polygastric Infusoria. 
It is much to be desired that we should possess as complete 
* Perhaps the most complete system is an apparently artificial one, inas- 
much as it draws the characters from one’single part. In all the Linnewan 
insects, the portions of the mouth present a sufficient variety to allow of the 
greater subdivisions being determined by that organ alone. All have origi- 
nally three pairs of jaw-bones. In insects, strictly so called, there is no 
passage from these to legs, but still apparently there are only two pairs pre- 
sent, for the third is united to the under lip. In the Arachnida, likewise, 
there are only two pairs apparently present, for the third is converted into 
the first pair of feet. In the Crustacea, on the other hand, there are appa- 
rently a larger number of pairs of jaws, inasmuch as the first (seldom the 
two first) or the three first pairs of feet, assume the form of jaws. In the 
Entomostraca there are three simple pairs of jaws, which lie in the opening 
of the mouth behind the first pair of feet. 
