188 INVERTEBRATE ZOOLOGY 



Class II. Gastropoda. Mollusks with, usually a distinct head 

 provided with feelers, a univalve, usually spiral shell, and a single 

 mantle cavity primitively posterior in position ; snails. 



Order 1. AMPHINEURA. Bilaterally symmetrical gastropods 

 with no shell or with a shell composed of eight transverse pieces. 

 Ex. Chiton. 



Order 2. SCAPHOPODA. Bilaterally symmetrical gastropods 

 with a cylindrical shell. Ex. Dentalium. 



OrderS. OPISTHOBRANCHIATA. Usually asymmetrical ma- 

 rine gastropods with posterior gills. Ex. Aeolis. 



Order 4. PULMONATA. Asymmetrical, freshwater or land 

 gastropods without gills, but with lungs. Ex. Helix. 



Order 5. PROSOBRANCHIATA. Asymmetrical gastropods with 

 anterior gills. Ex. Sycotypus. 



Class III. Cephalopoda. Mollusks with a large head bearing a 

 number of long arms. 



Order 1. TETRABRANCHIATA., Cephalopods with four gills 

 and a large convoluted chambered shell. Ex. Nautilus. 



Order 2. DIBRANCHIATA. Cephalopods with two gills and 

 either eight or ten arms ; squids. Ex. Loligo. 



Type IV. ECHINODERMATA. Metagastrozoans which are 

 radially symmetrical (being five-rayed) in the adult and have cal- 

 careous plates in the body-wall. 



Class I. Crinoidea. Mostly sessile echinoderms, with long vibra- 

 tile arms; sea lilies. Ex. Antedon. 



Class II. Asteroidea. Flattened, star-shaped echinoderms with 

 an ambulacral furrow and ambulacral feet on the underside of each 

 arm ; starfishes. Ex. Asterias. 



Class III. Ophiuroidea. Flattened echinoderms having long 

 vibratile arms without ambulacral furrows; brittle stars. Ex. 

 Amphiura. 



Class IV. Echinoidea. Spheroidal or flattened echinoderms with- 

 out arms ; sea urchins. Ex. Arbacia. 



Class V. Holothuroidea. More or less worm-like echinoderms 

 with oral tentacles ; sea cucumbers. Ex. Synapta. 



Type V. CHOKDATA. Metagastrozoans with a dorsal central 

 nervous system, an internal skeletal system consisting in the 



