TEN-LEGGED CRUSTACEA 419 



viii. Circulation. 



Thus, in the crayfish the organs of respiration are restricted to a few 

 definite portions of the body, i.e., the branchiae. Accordingly, as in 

 Vertebrata (see Part L, p. 7), bloodvessels must be developed by which 

 the (colourless) blood, loaded with carbonic acid, is conveyed to the gills, 

 and thence, after being purified, to the heart. (Contrast with insects.) 



C. Reproduction of the Crayfish, and Enemies. 

 i. Reproduction. 



Among crayfish offered for sale during winter and spring, we always 

 notice a number having eggs adhering to their abdominal feet (said to 

 be "in berry"). These are females. They carry the eggs about with 

 them until the young are ready to escape. In this manner they protect 

 them from numerous dangers, and also by constant movements of the 

 abdominal legs provide them with a constant supply of fresh water. 

 The latter is the more necessary, inasmuch as the females during this 

 period retire into their holes and but seldom go in search of food. The 

 young crayfish also for some time allow themselves to be carried about 

 by the mother, adhering by their pincers to the setae of its swimmerets. 

 This nurture of the brood also explains why the crayfish produces a much 

 smaller number of eggs than, e.g., fishes (see p. 277) ; and it also 

 furnishes it with a means of holding its own against its numerous 

 enemies. 



ii. Enemies. 



Amongst these, in addition to man (see Section A), we must class 

 the otter and water rat, the eel and the perch. Other means of protection 

 have been already dealt with, viz. : its concealment in holes, its colour, 

 the acuteness of its senses, its rapid movements when alarmed, its hard 

 shell and powerful claws. 



Other Decapod Crustaceans. 



Crustaceans fill in the water the place of insects on the land. They 

 inhabit the sea in immeasurable numbers and astonishing variety, and 

 are to man the source of a productive industry. 



Among the crustaceans of the coast of Europe, the Lobster (Homarus 

 vulgaris), which, however, is not found in the Baltic, is undoubtedly the 

 most important. This large creature, which may reach a length of 

 18 inches, hides under stones, in rock caverns, and among the tangle 

 of marine plants, on the look-out for such prey as may happen to swim 

 past, which it seizes by suddenly projecting its powerful toothed chelae. 



28 



