302 LECTURE XIV. 



abdominal ring, and is usually folded upon the back : it performs 

 the office of sweeping the gills, the flabellae, which have that function 

 in the Brachyura and Macroura, being absent from the branchial 

 chamber. 



The appendages of the abdominal segments {Jig. 128, l — 7) are de- 

 veloped, like those of the thoracic ones, from the inferior arcs : they 

 are usually broad, flat, ciliated plates, which may sometimes aid in re- 

 spiration, are more commonly organs for swimming, form a temporary 

 place of attachment and protection to the ova, and are restricted to 

 this use in the females of the Brachyura, in which the abdomen or 

 tail is concealed by being bent forwards upon the sternum. In the 

 hermit-crabs, which present an anomalous softness of their abdominal 

 segments, the last pair of appendages form the claspers by which the 

 parasite holds fast by the columella of the shell it may have selected 

 for its abode. 



The calcified integument of the Crustacea being inelastic, inex- 

 tensile, and not endowed with inherent powers of growth ; being 

 likewise so disposed in the Podophthalma as to inclose and protect 

 the greater part of the body by annular or large shield-like pieces 

 like back- and breast-plates, must needs be thrown off to allow of the 

 growth of the animal. This moult of the external integument has 

 been observed in a few species of Crustacea, in the full-grown 

 specimens of which it appears to take place annually ; and a like 

 ecdysis is probably common to all the class. Reaumur gave the first 

 good account of the process in the craw-fish (Astacus fluviatilis). 

 It takes place generally in the month of August. The animal pre- 

 viously retires to some place of concealment, is quiet and fasts for a 

 few days, during which time the old crust is loosened from the lining 

 membrane beneath, which becomes thickened, vascular, and like velvet. 

 As this begins to harden, the animal takes measures to rid itself of its 

 old crust ; it rubs its legs against each other, throws itself upon its back, 

 contracts and swells out the body, with alternate violent inflections 

 and intervals of rest. The carapace is thus separated from the abdominal 

 segments, is pushed upwards, and the animal liberates its head with 

 the eyes and antennaj, which are provided with new sheaths : then 

 the more difficult operation of freeing its extremities takes place : 

 finally, the craw-fish creeps out of the remainder of its old shell by 

 withdrawing the abdominal segments ; when the parts of the cast-off 

 shell frequently return so nearly to their old positions as to represent 

 the outward form of the animal with all its appendages, even the 

 hairs and lining of the stomach. In one or two days the calcification 

 of the new crust is completed, and the animal is restored to health 

 and activity. During this period two very remarkable accumu- 



