Because giant squids can attain a lengtti of 

 fifty feet or more, ttiey tiave earned ttieir 

 reputation as "sea monsters." A ten-armed 

 ceptialopod, the squid, iit<e ttie octopus, 

 is a moliusc ttiat has iost its outer shell 

 during the course of evolution. 



since its Latin root is mollis (soft). Obviously, there are plenty of 

 animals with even softer bodies than those of molluscs. What is 

 actually implied in the name is that the molluscs have a soft body 

 that contrasts with the hardness of the shells in which they live. 

 Typically the shell is of carbonate of lime and conforms pretty 

 much to one of two patterns. On the one hand we have the bivalves 

 (hinged shells), such as clams and oysters; on the other, the uni- 

 valves (spiral shells), such as snails and slugs. But there are several 

 modifications of these two basic patterns : The shell may be lost 

 altogether, it may be reduced in size or altered in shape, or it may 

 be laid down inside instead of outside the body. So classification of 

 the molluscs cannot be based wholly on the appearance of the shell. 



There are, in fact, five classes of molluscs. In one, the Amphi- 

 neura, belong the chitons or coat-of-mail shells. These range in 

 length from half an inch to ten inches, and all have a flat muscular 

 foot and carry on their backs eight overlapping crescent-shaped 

 plates that represent the shell and suggest some kind of segmenta- 

 tion. As the most primitive of molluscs, the chitons are close to the 

 bristle worm. The next class, the Scaphopoda, is little known and 

 comprises the tusk shells, whose shells resemble elephant tusks. 

 Next come the Gastropoda (snails and slugs on land, whelks and 

 sea snails in the oceans). Like the chitons they have a muscular foot 

 and, usually, a spiral shell. But some, Uke the limpets, have a conical 

 shell, and others, Uke the sea slugs, have none at all. Then comes 

 the most numerous class, called Pelecypoda, consisting of the bi- 

 valves of which the most popular examples are oysters, mussels, 

 and clams. Finally, there are the Cephalopoda, those remarkable 

 invertebrates including the octopus, squid, and cuttle. Some of 

 these have lost their shell entirely, others carry only the relics of a 

 shell inside their body. Yet the related nautilus has a well-grown 

 spiral shell. The cephalopods are far more active than the four 

 other classes of molluscs. Speedy hunters, they hold their prey in 

 their arms, which are lined with suckers, and eat it with their 

 parrotlike beaks. Most remarkable of all, perhaps, is the eye of the 

 cephalopods, which is as highly developed as that of many higher 

 vertebrates. 



So much for the lower invertebrate sea dwellers. As for the 

 higher invertebrates, we have already noted that they are mostly 

 land creatures. Called arthropods because of their jointed legs, only 

 one big class — the Crustacea — plus fifty or so types of insect 

 inhabit the oceans. The crustaceans have jointed bodies as well as 

 jointed legs, the body being clothed in a chitinous (horny) cuticle 

 which may or may not be reinforced with lime salts. This cuticle is 

 soft at the junctions between the segments — a softness that enables 

 the lobster, for example, to flex its tail. On the head of every 

 crustacean are two pairs of antennae ; and nearly every segment of 

 the body has one pair of legs for swimming or walking. 



Of the five subclasses of crustaceans the first includes the fairy 

 shrimps and the water fleas that live in fresh or brackish water. 

 These are tiny creatures, as are the ostracods, whose distinguishing 

 mark is a bivalve shell formed by the cuticle. The third subclass, 

 the copepods, has a typically spindle-shaped body with a forked 

 tail, several pairs of swimming legs, and two pairs of antennae, one 

 of which is noticeably the longer. The fourth subclass, the barnacles, 

 needs no introduction. The ones many of us know best are the 



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