SCAPHOPODA. 237 
colored by carminate or ammonia, or Prussian blue dissolved in oxalic 
acid, or the precipitate of chromate of lead, or he even injected air 
into the vascular cavities. The mollusk should, before injection, be 
allowed to slowly die for several days, and the fluids to leave the body. 
The injection should be made before decomposition has set in, otherwise 
the vessels will burst. Some anatomists plunge mollusks into water 
to which has been added alcohol and chlorhydric acid. After remaining 
in this fluid for a day or two they can be injected. The arterial system 
can best be injected by the aortic bulb, or aorta; the venous system 
may be filled from the foot through the aquiferous orifice, by the 
adductor muscle, or by any of the large veins. After injection the 
animal should be plunged into cold water to hasten solidification and 
then placed permanently in alcohol. 
Oxass II.—CrpHaLopHora (Whelks, Snails, etc.). 
General Characters of Cephalophores.—We now come to 
Mollusca with a head, distinguishable from the rest of the 
body, bearing eyes and tentacles ; but the bilateral symmetry 
of the body, so well marked in the Acephala, etc., is now 
in part lost, the animal living in a spiral shell ; still the foot 
and head are alike on both sides of the body; while the 
foot forms a large creeping flat disk by which the snail glides 
over the surface. Moreover, these mollusks have, besides 
two pharyngeal teeth, a lingual ribbon or odontophore. In 
a shelless land-snail (Onchidium) Semper has discovered the 
existence of dorsal eyes, constructed, as he claims, on the 
Vertebrate type.* They are in the form of little black dots 
scattered over the back of the creature, and their nerves 
arise from the visceral ganglion. Familiar examples of the 
Cephalophora are the sea-snails, the sea-slugs, and the 
genuine air-breathing snails and slugs. 
Order 1. Scaphopoda.—A very aberrant type of the class 
is Dentalium, the tooth snail, common in the ocean from 
ten to forty fathoms deep, on our coast. It lives in a long 
slender tooth-like shell, open at both ends, while the animal 
has no head, eyes, or heart, and the foot is trilobed. Owing 
to the presence of a lingual ribbon, we would retain it in the 
present class, though it is a connecting link between this and 
* Over 10,000 ‘‘ eyes,” or sense-organs, occur in the shell of Chiton. 
