194 DECAPOD CRUSTACEA. PART m. 



nourished the different organs, it is collected in reser- 

 voirs at the base of the gills, is aerated while passing 

 through them, and returns to the heart again. 



The mouth opens through a short gullet into a large 

 globular stomach, from the walls of which calcareous 

 toothed organs meet in the centre. One serves as an 

 anvil, while the others bruise the food on it. Some of 

 the long-tailed crustaceans can evert this apparatus and 

 push it out of their mouth. The bruised food is lique- 

 fied by solvent juices from the liver and stomach, and 

 the nutritious part enters the bloodvessels by imbi- 

 bition. 



The nervous system is condensed to suit the form of 

 the crab. An oval nervous mass with a hole in its centre 

 surrounds the gullet, from each side of which a nerve 

 extends to a nerve-centre in the head. The organs of 

 sense are as usual supplied with nerves from the latter, 

 and, from the circumference of the massy ring, nerves 

 radiate to every part of the animal, voluntary or reflex, 

 as may be required. 



Dr. Carpenter has proved, by microscopic observations, 

 that the shell of the Decapod, in its most complete form, 

 consists of three strata : the first is a horny structureless 

 layer covering the exterior ; the second, a cellular stra- 

 tum ; and the third is a laminated tubular substance. 

 In the large, thick -walled crabs, as the Cancer pagu- 

 rus, the three strata are most distinctly marked. The 

 tubuli of the lowest layer rise up through the pigment 

 stratum in little papillary elevations, which give the 

 coloured parts of the shell a minutely speckled appear- 

 ance. There are various deviations from this general 

 plan. In many of the small crabs belonging to the genus 

 Portunus, the whole substance of the shell below the 

 structureless horny investment is made up of hexagonal, 

 thick- walled cells; and in the prawns there are large 

 stellate coloured cells. 



