264 



THE RISE OF ANIMAL LIFE 



Fig. 12-14. The nudibranch gastropods are without gills 

 and breathe through their skin, which is often thrown 

 up into papilla-like structures that function somewhat 

 as gills do. Those pictured here (Hermissenda crassicor- 

 nis) are yellow-green in color and are about an inch 

 long. 



The two specimens in the top picture are feeding on 



sites themselves. In former times snails 

 had some value as a source of food, but this 

 is negligible today. 



The slugs are gastropods that do not bear 

 shells. The nudibranch, prosaically called 

 the sea slug, does have a shell during the 

 larval stage but the adult form appears to 

 be a snail without a shell (Figs. 12-14, 12- 

 15). The name slug was apparently given 

 to the lifeless preserved laboratory speci- 

 men, which takes on a dingy collapsed ap- 

 pearance after it has been exposed to light 

 and preservative. When observed in its 

 natural habitat on rocky coasts, however, 

 these colorful animals are found to be most 

 inappropriately named. 



Although the gastropods were of eco- 

 nomic significance to ancient man, the 

 pelecypods serve modern man to the great- 

 est extent. Clam chowder, sauteed scallops, 

 and oyster cocktails have become favorite 

 forms of sea food all over the world. The 

 shells of the bivalved animals are also used 

 by man. Most cherished are pearls, the rare 

 jewels secreted by the mantle of the sta- 

 tionary fresh-water clams and pearl oysters. 

 The bits of shell that are cut and polished 

 into buttons are products of fresh-water 

 bivalves, the clams. 



Members of the class Cephalopoda are 

 the most highly organized of the mollusks 

 and include the largest species of the in- 

 vertebrate animals. The head region, as the 

 name implies, is large and well developed, 

 unlike most of the preceding groups. Most 

 forms of this class bear two large complex 

 eyes, resembling the eyes of vertebrates. 

 Some have continuous shells, such as the 

 shell of the Natitilm (Fig. 12-16), a mem- 

 ber of the group immortalized by Oliver 

 Wendell Holmes in his poem, "The Cham- 

 bered Nautilus." In others, such as the 



hydroids which they are able to do without discharg- 

 ing the nematocysts. In fact they incorporate the 

 stinging cells into their own body to be used some- 

 time later for their own defense. Just how they do 

 this is unknown. 



Another specimen, pictured below, is laying its long 

 strings of eggs. 



