418 TEXT-BOOK OF ZOOLOGY 



by muscles, as anyone knows who has eaten crayfish or lobsters. (Com- 

 pare with the tail of fishes.) 



(b) As great mobility is required for the execution of the strokes, we 

 find that the abdominal somites are distinctly separated from each other. 

 (Contrast this with the cephalo-thorax.) 



(c) Laterally the somites are widened out, and are curved in such a 

 manner that the whole abdomen forms a scoop which exerts considerable 

 force in striking the water. 



(d) The so-called tail fin, also effects a considerable enlargement of 

 surface. It consists of five plates, of which the central one represents 

 the last abdominal somite (since it carries the anal aperture), whilst the 

 lateral plates are the much-expanded branches (exopodite and endopodite) 

 of the appendages of the penultimate (sixth) abdominal somite, similar 

 to those attached to the under surface of the other segments of the 

 abdomen (see Section C, i.). 



vii. Respiratory Organs. 



The crayfish, as a typical aquatic animal, breathes, like the fish 

 (see Part II., p. 272), by gills or branchice. These lie at the sides of the 

 carapace in the so-called branchial chamber, and consist of very fine plumes 

 and filaments attached to the second and third pairs of foot-jaws and to 

 the first four pairs of walking legs. These delicate organs are protected 

 by a cover formed by the sides of the cephalo-thorax, analogous to the 

 gill-cover in fishes, and, just as in the latter, the branchiae require to be 

 bathed by a constant stream of water. If we watch the under surface of 

 a crayfish which has been introduced into a glass vessel, we shall see 

 that the plate-like second pair of maxillae vibrate rapidly to and fro, and 

 also that the flagelliform appendages of the two last foot- jaws are in 

 constant motion. If a small quantity of a not easily soluble pigment 

 (e.g., carmine) be now added to the water, we observe that the grains of 

 pigment are swept away from the vibrating mouth parts, showing that 

 the respiratory* current is directed from behind forwards, thus corre- 

 sponding to the direction assumed in swimming. 



Dirt particles, if allowed to be deposited upon branchiae or to block 

 up the outflow from the branchial chamber, would impede the function 

 of these delicate organs. To prevent this, the free edge of the gill-cover, 

 which lies close up against the body, is beset with fine hairs, and, like a 

 strainer, holds off all solid constituents of the inflowing water from the 

 branchial chamber. Hence, also, it is necessary that the water inhabited 

 by the crayfish should be clear and the bottom not covered with loose 

 mud, which would be constantly stirred up by the animal while striding 

 through it. (Why can crayfish live for a considerable time out of the 

 water? Compare with fishes, Part II., p. 274.) 



