MOLLUSCA I2Q 



sacs at their bases ; two eyes, which are often on stalks ; 

 and a . straplike tongue (odontophore), covered with 

 minute teeth (Fig. 227). The heart is situated, in the 

 majority, on the right side of the back, and consists of 

 an auricle and a ventricle (Fig. 243). The nervous 

 ganglia are united into an esophageal ring or collar 

 (Fig. 351). All, except the pteropods, move by means 

 of a ventral disk or foot. 



Gastropods are now the reigning mollusks, comprising 

 three fourths of all the living species, and are the types 

 of the branch. They have an extraordinary range in 

 latitude, altitude, and depth. 



Omitting a few rare and aberrant forms, we may sepa- 

 rate the class into the following orders : 



1. Aspidobranchia, gastropods having a somewhat 

 diffuse nervous system, the cerebral ganglia being wide 

 apart, two auricles in the heart, gills plumelike, limpet 

 (Patella, Fig. 105), well known to every seaside visitor, 

 and the beautiful ear-shell (Haliotis, Fig. 95), frequently 

 used for ornaments and inlaid work, the pyramidal 

 Trochus, and the pearly Turbo (Fig. 102). 



2. Pectinibranchia, gastropods with a somewhat con- 

 centrated nervous system, heart with a single auricle, 

 gill bearing a single row of lamellae and attached to the 

 wall of the mantle. This order includes many of the 

 most beautiful of the sea shells, the cowry (Cyprcza) 

 (Fig. 94), cones (Fig. 99), whelk (Buccinum) (Fig. 88), 

 trumpet shell (Triton), volute (Fig. 101), olive, harp, 

 cameo shell (Cassis) (Fig. 97), rock skell(Mupex), spindle 

 shell (Fus2is) (Fig. 96), and wing shell (Strombus)(ig. 

 103). All of these are marine. Many of them are 

 carnivorous and have the margin of the shell notched. 



3. Opisthobranchia. The pteropods are small, marine, 

 floating mollusks, whose main organs of motion resem- 

 ble a pair of wings or fins, coming out of the neck, 



DODGE'S GEN. ZOOL. 9 



