houses, which now are empty (Fig. 32-29). 



Locomotion among Mollusca also varies 

 widely. Oysters, mussels, and related forms 

 attach themselves to the substratum (Fig. 32- 

 29) and do not move at all during their 

 adult life. Many clams merely burrow into 

 the sand, while others, such as the common 

 edible hard-shelled clam (Venus mercenaria), 

 plow along half buried in the sand, slowly 

 extending and retracting the muscular toot 

 (Fig. 32-29). Snails, slugs, whelks, and chitons 

 (Fig. 32-29) creep along the substratum, using 

 an elongate muscular foot, which is closely 

 applied to the surface; and the octopus 

 quickly clambers over a rocky surface, or 

 slithers into and out of a dark crevice by 

 means of the eight powerful tentacles of its 

 highly modified foot. Squids dart forward 

 and backward with flashing speed, using a 

 sort of jet propulsion. The mantle around 

 the body in these animals has become modi- 

 fied into a strong conical muscular sac (Fig. 

 32-29). When this sac expands, water enters 

 the mantle cavity through the incurrent 

 siphon. Then when the sac contracts forcibly, 

 a strong jet of water shoots out through the 

 excurrent siphon, propelling the animal — in 

 a forward direction if the siphon is pointed 

 backward, or in a backward direction if the 

 siphon is pointed forward. Moreover, the 

 squid guides and steadies its darting move- 

 ments with fins, at the posterior end of the 

 body, and with its anteriorly placed tentacles 

 (Fig. 32-29). 



Phylogenetic Relationships. The rich fos- 

 sil record of the Mollusca includes more than 

 35,000 extinct species, localized in all strata 

 from the late Cambrian period to recent 

 times. Nevertheless, the origin of the group 

 remains obscure. The primitive molluscan 

 stock of pre-Cambrian times was devoid of 

 skeletal parts and seems to have left no 

 record. The embryological evidence, based 

 on a comparative study of the cleavage pat- 

 terns and early larval stages, indicates some 

 relationship to the Annelida. Adult Mol- 

 lusca, however, are quite different from 

 Annelida, especially as to their lack of seg- 



The Animal Kingdom - 653 



mentation and their reduced coelomic cavity. 

 Probably the two groups separated from each 

 other during pre-Cambrian time and the 

 classes of Mollusca became differentiated dur- 

 ing the Ordovician, Devonian, and Silurian 

 periods (Fig. 29-11). 



Classes of the Mollusca. The Mollusca 

 have evolved as five distinctive classes, as fol- 

 lows: 



Class 1. The Amphineura (Literally, "Two- 

 Nerved"). Chitons (Fig. 32-29) and related 

 forms; body elongate or oval; shell, when 

 present, consisting of eight serially arranged 

 valves; head, reduced, or absent; two pairs of 

 ventral nerve cords; a relatively small exclu- 

 sively marine group (650 living, 75 fossil 

 species). 



Class 2. The Scaphopoda (Literally "Boat- 

 footed"). The "tooth-shells" such as Denta- 

 lium (Fig. 32-29); shell slenderly conical, open 

 at both ends; foot also cone-shaped; another 

 small marine group (200 living, 300 fossil 

 species). 



Class 3. The Gastropoda (Literally, "Stom- 

 ach-Footed"). Snails, slugs, limpets, and 

 whelks (Fig. 32-29); shell spirally coiled and 

 univalvular, except in a few forms; distinct 

 head; foot is large, flat, and ventrally placed, 

 in contact with substratum; a large group 

 (30,000 living, 13,000 fossil species); mostly 

 marine, but with some fresh-water and ter- 

 restrial forms. 



Class 4. The Pelecypoda (Literally, "Hatchet- 

 Footed"). Clams, oysters, scallops, and other 

 bivalved mollusks (Fig. 32-29). Two valves of 

 shell placed laterally, hinged by dorsal liga- 

 ment and closed by one or two adductor mus- 

 cles; no head and almost always lacking a 

 radula in the mouth; a medium-sized aquatic 

 group, mainly in salt water (10,000 living, 

 14,000 fossil species). 



Class 5. The Cephalopoda (Literally "Head- 

 Footed"). Octopuses, cuttle fish, squids, and 

 nautiluses (Fig. 32-29). Shell external, in- 

 ternal, or lacking; "head-foot," bearing ten- 

 tacles (ten in squids, eight in octopuses); eyes, 

 conspicuous and highly organized; mouth, 

 with horny beaklike jaws,- in addition to a 



