90 ANIMAL BIOLOGY 



finally, pearls produced by the irritated mantle tissue of several 

 species of Bivalves are perhaps the most highly prized and priced 

 jewels. 



Cephalopoda. The Squids, Cuttle-fish, Devil-fish, and their 

 close allies present in many ways a marked contrast with the rest 

 of the Molluscs, being relatively active, aggressive animals that 

 have in most cases practically discarded the shell and developed 

 a highly specialized 'head' apparently by combining head and 

 foot — hence the class name Cephalopoda. (Fig. 50.) 



The head, surrounded by arms, or tentacles, is provided not 

 only with parrot-like beak and rasping tongue, but also with a 

 rather large brain, and efficient eyes that superficially are very 

 like those of Fishes. As a matter of fact some of the Cephalopods 

 are in many respects more capable than some of the lower Verte- 

 brates. They apparently exhaust the possibilities of the Molluscan 

 body plan both in complexity and in size. Indeed, the Giant Squid 

 is the largest Invertebrate, for when measured to the tip of the ex- 

 tended arms it exceeds in length any living Vertebrate except the 

 largest Whales. 



I. Arthropods 



From our detour through the unsegmented Molluscan phylum, 

 we now turn to the more orthodox — because dominant — phylum of 

 segmented Invertebrates with jointed appendages, the Arthrop- 

 oda. The most important classes of the Arthropoda are the Crusta- 

 cea, Myriapoda, Insecta, and Arachnoidea: the first chiefly aquatic 

 and breathing by means of gills, and the rest typically terrestrial 

 with tracheae or equivalent air-breathing organs. (Fig. 116.) 



Crustacea. A varied multitude of marine and a relatively 

 small number of fresh-water animals constitute the class Crustacea. 

 Among its approximately twenty thousand species, probably the 

 best known, because they are among the largest and some are 

 edible, are the Lobsters, Crayfishes, and Crabs. The latter appear 

 unique because the posterior part of the body (ardomen) is 

 permanently bent forward under the cephalothorax. Then 

 at the other extreme, as it were, are the little-known Wood-lice, 

 or Pill-bugs, that live in damp places, even in our gardens. All of 

 these and their many close relatives form one of the two Crustacean 

 subclasses, the Malacostraca. The other great subclass, the 

 Entomostraca, includes even greater diversity of structure. 



