ARTHROPOD A 2 93 



The barnacles (Cirripedia) are Crustacea which in adult life become attached 

 to the rocks near' low water-mark or to floating objects of various kinds. The 

 bottoms of ships become foul with them. The group is especially interesting in 

 that it points to the giving up of free motion, which its ancestors possessed, for a 

 mode of life wholly different, and demanding marked changes of structure. They 

 possess, besides the organs for attaching themselves, bivalve shells similar to those 

 of Mollusca, for protection; they are often hermaphroditic, which is a very un- 

 common thing in arthopods. The advantages gained by their special habits are 

 evident. The waters near the shore contain a great deal of organic de"bris, and any 

 organism which can attach itself here and yet be protected from destruction by 

 the waves is fortunate. Those attached to floating objects are brought, without 

 their effort, into constantly changing localities. 



Subclass II. Malacostraca. Crustacea of larger size and more highly organized. 

 Segments, except in one order, twenty, and well differentiated. Nineteen of 

 these segments bear appendages. The first stage in the metamorphosis (the 



PIG. 131. 



FIG. 131. Larva of Lobster (Homarus americanus) removed from egg shell. From Herrick. 



Questions on the figure. Compare with the adult (Fig. 132) and note similari- 

 ties and differences? Examine Dr. Herrick's figures (Bull. U. S. Fish Commission 

 for 1895) and notice the gradual change to the adult condition by successive 

 moultings. What structures can you identify? 



nauplius) is usually passed before hatching. The group embraces (i) the Deca- 

 poda, or the lobsters, crabs, crayfishes and shrimps, which agree in the possession 

 of ten walking feet, eyes on movable stalks and a carapace covering the thirteen 

 fused segments of the cephalothorax ; and (2) the Tetradecappda, comprising 

 numerous smaller types such as beach-fleas, sow-bugs or wood-lice, in which head 

 and thorax are not fused, the eyes are not movable, and the walking appendages 

 are fourteen. 



The crayfish and lobsters have well-developed abdominal segments, whereas 

 in the crabs the abdomen is reduced and bent under the thorax, which becomes 

 broad and massive (Figs. 129, 130). The larger Crustacea are omnivorous, almost 

 all organic matter, dead or living, being acceptable. Lobsters are known to attack 

 and devour fishes. The lobster (Homarus, Figs. 131 and 132), of which there are 

 two species, an American and a European, is economically the most important 

 member of the group, and stands next the oyster as the most important invertebrate 



