122 UNDERWATER GUIDE TO MARINE LIFE 



or king helmet CColor Plate JO). Helmets prey chiefly on bivalve molluscs and 

 reach a size almost as large as that of the queen conch. 



The toxoglossans include the poisonous cone shells, Conns (^Color Plate 10^, 

 which have the radula reduced to two long, sharp lancets attached to a poison 

 gland. The cone shells are not known to be dangerous in North America, but 

 some of the South Pacific species are deadly. They should not be handled. 

 Treatment of the poisoning is just like that for snakebite (consisting of lancing 

 the wound in order to encourage bleeding, followed by immediate attention 

 by a physician). Cone shells can always be recognized by their conical shape. 



The remaining groups of snails all tend to reduce the shell. Pteropods are 

 seldom more than Vi of an inch long, have transparent, papery shells, and 

 are planktonic. Their foot is divided into two large "wings" for purposes of 

 swimming. They are common enough in some places to form bottom oozes from 

 their cast-off shells. The large sea slug or sea hare, Aplysia QcoJor photograph^, 

 reaches almost the size of a football, feeds on algae, and ejects a harmless, inky 

 fluid when it is molested. The smaller plumed sea slugs or nudibranchs CColor 

 Plate W) are probably the most beautifully colored of all invertebrates. Their 

 soft but iridescent colors and flowing movement are extremely beautiful. They 

 grow to a maximum of about 4 inches. Most plumed sea slugs are of cold-water 

 distribution, but some species are also found in the tropics. They are carni\'orous, 

 and some are almost unique in that they eat sponges which are avoided by all 

 other animals but sea spiders. Other species eat anemones or bite the heads 

 off^ hydroids for food. (The hydroids do not seem to be bothered much by 

 this since they quickly grow a new head.) The plumes on the backs of sea 

 slugs are used as respiratory gills and have given these animals the name of 

 "nudibranch" which means "naked gills." One nudibranch, Eolis CColor Plate 

 lOX has extensions of its digestive tract reaching up into its dorsal plumes. 

 The nematocysts of the coelenterates, which it eats, are stored in these exten- 

 sions and are used by Eolis for defense— a completely unique use of the 

 defensive mechanism of one animal by another. Eolis's plumes are brightly and 

 warningly colored, and Eolis is avoided by fishes. 



Tooth Shells: Class Scaphopoda — Figure 40 



These litde animals have a long hollow shell, a radula like the snails, and a 

 burrowing foot like the clams. They never are more than about a half-foot long 

 and are usually half that length. They burrow into sandy or muddy bottoms 

 and feed on microscopic organisms. They are chiefly of cold or deep waters 

 offshore. 



Bivalves: Class Pelecypoda — Figure 41 



These are compressed animals with no head and with two shells or valves 

 united by a hinge. The hinge is elastic, and when the shells are closed by 

 muscles, the hinge is compressed. When the muscles are released, the hinge 

 causes the shells to open. Thus, these animals must exert muscular force to 

 keep their shells closed but exert no force to open them. A starfish is able to tire 

 a bivalve merely by a continued pulling force. When the much stronger bivahe 

 relaxes its muscles out of exhaustion, the starfish is able to eat it. 



