8GAPHOPODA. 257 



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. 



CLASS II. CEPHALOPHORA (Wielks, 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 Acepliala, 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 

 Ceplialopliora, 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 



