ABERRANT ANIMALS-THE MOLLUSKS AND ECHINODERMS 265 



Fig. 12-15. This is another beautifully colored naked gastropod {Phyllaplysia taylori) found in the oceans. Note the 



delicate lines and the two tentacles that protrude like horns. 



cuttlefish (Sepia) and the squid, the shell 

 is located internally. In addition to a gen- 

 erally large, fleshy body, the cephalopods 

 usually have long muscular arms, or 

 tentacles, which are modified portions of 

 the foot. 



Fig. 12-16. The chambered nautilus, a cephalopod mol- 

 lusk, lives in the deep waters (1800 ft.) of the South 

 Pacific. Its shell, shown here, is made up of many 

 chambers, the last and most spacious being occupied 

 by the animal. 



One cephalopod, the squid, is not only 

 unique in organization but also in the wide 

 range of sizes in which it occurs (Fig. 12- 

 17). Different species of squids vary from 

 miniature animals 1 inch long to giant 

 forms of 18 feet, or twice that length when 

 the arms are stretched out. Fossil remains 

 show that the squid was one of the most 

 prominent animals during prehistoric 

 times. The tapering body of this mollusk, 

 which suggests an arrowhead or rocket, 

 enables it to shoot through the water with 

 lightning-like speed, either forward or 

 backward, changing its direction simply by 

 directing its ventral siphon toward or away 

 from the anterior arms (Fig. 12-18). This 

 and the medusa are the only animals to use 

 jet propulsion in locomotion. The squid is 

 also equipped with fins to aid in swimming 

 and directing its course through the water. 



Even though the squid is a rapid swim- 



