CRUSTACEA. 429 



A cmstaceous animal consists of three parts : the head, 

 the body, and the carapace, which is covered with one 

 entire shell, and is popularly called the tail, consisting of 

 seven rings, or joints. There are properly fourteen rings 

 in that part of the body which is called the carapace ; but 

 they are only used when the animal changes its shell. The 

 joints in the tail are to enable the animal to spring for- 

 ward, which it does frequently when it wishes to change its 

 position. It can also crawl ; but it moves in this manner 

 awkwardly, and in an oblique direction. The Shrimp, 

 though it has no claws, properly so called, has two feet 

 larger than the rest, each of which has a hooked jointed 

 process at the extremity. The Prawn differs from the 

 Shrimp, and is nearly allied to the Crawfish, or thorny 

 Lobster. Prawns (Paloemon), Shrimps, and Sand-raisers 

 (Crangon), and Thalassinadce } burrow in the sand, mostly 

 in shallow water. Sand-hoppers (Talitrus) are found in 

 great numbers on the beach ; whilst the Podosomata and 

 the smaller Entomostraca are found in the tufts of sea- 

 weeds that grow between tide levels. All the Crustacea 

 have the power of renewing their claws, if they are torn off 

 at a joint, and they change their shells every year. The 

 development of the Crayfish (Astacus) has been the subject 

 of an admirable memoir by Eathke. The apparatus which 

 is commonly regarded as the auditory organ was first 

 demonstrated by Dr. Arthur Farre. He says : " The organs 

 connected with the external antennae, and considered by 

 anatomists generally to have an auditory function, are not 

 the true organs of hearing ; but that they are situated in an 

 oval space, on the largest joint of the antennule. On cut- 

 ting away the hairs close to their base, they are found to 

 cover a funnel-shaped aperture of about one-sixteenth of 

 an inch long. A nerve can be traced to the base of this 

 canal. The eyes are divided into a great number of minute 

 quadrilateral facets, each of which corresponds with the 

 base of a quadrilateral pyramid, constituted by a mem- 

 branous sheath, containing the clear vitreous humour, 

 whose apex is prolonged towards the bulbous expansion of 

 the optic nerve. The faces of the pyramids are separated 

 throughout their whole length by a dark pigment, which 

 performs the function of a choroid coat." 



