Morse. ] ys [March 19, 
sented sufficiently indicate the wide divergence of the two great 
Divisions. 
Note. The annulated embryo of the worms is characteristic of most of them, 
from the Rotifer to the highest Cheetopod. In all, the body is generally divided 
into a few transverse segments. In the Lamellibranchiate and Gasteropod the 
embryo early develop the velum, or foot, projecting from a bivalve, or a nautiloid 
shell. In Chiton the larva is annulated, according to Loven. Pneumodermon, 
among the Pteropods, has the body banded by transverse circles of cilia. In 
Dentalium the larva resembles that of a true worm. 
The affinities of Dentalium are not clearly understood. It was placed among 
the Annelids by Cuvier and Lamarck, and then among the Mollusks by Deshayes 
and De Blainville, as Gasteropods. Since then they have been bandied about 
from one end of the series to the other. Lacaze-Duthiers,1 who has made the 
most thorough investigation of them, makes a separate class, Solenoconchia, 
with their affinities mostly among the Lamellibranchiates. Huxley places them 
with the Pteropods, on account of the rudimentary head, neural flexure of in- 
testine, presence of epipodial lobes, and the character of the larva. With all 
these diverse relations, I would suggest that they certainly bear some relations 
to the Tetrabranchiate Cephalopods, in the numerous and retractile tentacles, 
the dorsal turn of the shell, and the strict identity between a peculiar bilateral 
cartilaginous body which occurs in the head of Dentalium, as well as in the 
head of Nautilus pompilius. 
Having thus connoted the leading features characteristic of each 
Division, our next object is to inquire whether all the characters 
of major and minor importance possessed by the Brachiopods are not 
held in common by the worms, and are in no wise possessed by the 
Mollusks. 
General Proportions of the Body. 
In Mollusks, while we may have the body divided into a creep- 
ing disk, and a visceral portion, the visceral portion usually car- 
ried above in a protruding chamber (See Fig. 3), or the mantle 
prolonged behind, to form the tubes, we do not have the body 
constricted transversely, forming a thoracic, and an abdominal por- 
tion. We do not find such a feature as a caudal appendage, 
nor are the Mollusks ever attached, save by the adhesion of the 
calcareous shell, or by the byssus. Among the lower worms, as, for 
example, some Rotifera, certain forms are fixed by their posterior 
portion. In the Rotifera, as well as in the tubicolous Chetopods, 
1 Annales des Sciences Naturelles, 1856-57. 
