1873.] 31 9 [Morse. 
A typical Lamellibranch, and a typical Gasteropod, will be ad- 
mitted by all, as best representing this branch, for while other groups 
have widely changed since their first appearance in past ages, we 
find the Lamellibranch and Gasteropod of the lower Silurian as typi- 
eal as present existing forms, e. g., Modiolopsis, Avicula, Murchi- 
sonia, Pleurotomaria, and from the tracks and tubes, and still later 
setze, we are safe to assume that the Annelids were as characteristic 
of their classes in past geological times, as at present. 
We cannot compare the Trematodes and Turbellarians, with the 
Nudibranchiate Mollusks, for however much resemblance some may 
see in theiradult condition, as among the Planarians,! certain charac- 
ters of external symmetry in common, their respective embryos are 
identical with their respective divisions, the one being annulated, the 
other developing a foot, and a nautiloid shell. 
Leaving these out of consideration, then, and taking the dominant 
characters displayed by the Vermes on the one hand, and the Mollusca 
as cited on the other, we have in the Vermes, a form, whose length is 
much greater in proportion to its breadth than in the Mollusks; the 
Fig. 1. Fig. 2. 
Transverse section of Annelid Transverse section of molluscan 
after Carus. archetype after Carus. 
b. bands suspending intestine in perivisceral cavity. 7. intestine. s. segmental 
Organ. sé, sé. sete. 
worms being drawn out as it were, the Mollusk being concentrated. 
The worm is perfectly bilaterally symmetrical, depressed, flattened 
or circular, the dorsal and ventral regions so near alike in many cases, 
as to be distinguished with difficulty, and the body never flattened 
laterally. The Mollusk is also bilateral, but often asymmetrical, 
the dorsal and ventral regions are very unlike, and the body almost 
always flattened laterally. This latter character is so marked, more 
especially among the Lamellibranchiata, as to have led Prof. Agassiz 
1 Girard placed the Planarians with the Mollusks. 
