METAMORPHOSES OF DECAPODS. 



551 



minutely speckled appearance. Many departures from this type 

 are presented by tlie different species of Decapods ; thus in the 

 Prawns there are large stellate pigment-cells (resembling those 

 of Fig. 327, c), the colors of which are often in remarkable con- 

 formity with those of the bottom of the rock-pools frequented by 

 these creatures ; whilst in the Shrimps there is seldom any dis- 

 tinct trace of the cellular layer, and the calcareous portion of the 

 skeleton is disposed in the form of concentric rings, an approach 

 to which arrangement is seen in the papillae of the surface of the 

 deepest layer of the Crab's shell. 



375. It is a very curious circumstance, that a strongly marked 

 difference exists between Crustaceans that are otherwise very 

 closely allied, in regard to the degree of change to which their 

 young are subject in their progress towards the adult condition. 

 For whilst the common Qrah, Lobster, Spiny Lobster, Prawn, and 

 Shrimp, undergo a regular metamorphosis, the young of the 

 Land Crab and the Cray-fish come forth from the egg in a form 

 which corresponds in all essential particulars with that of their 

 parents. Generally speaking, a strong resemblance exists among 

 the young of all the species of Decapods which undergo a meta- 

 morphosis, whether they are afterwards to belong to the brachy- 

 ourous (short-tailed) or to the macrourous (long-tailed) division of 

 the group ; and the forms of these larvae are so peculiar, and so 

 entirely different from any of those into which they are ultimately 

 to be developed, that they were considered as belonging to a 

 distinct genus, Zoea, until their real nature was first ascertained 

 by Mr. J. V. Thompson. Thus in the earliest state of Oarcinus 

 mcenas (small edible crab), we see the head and thorax, which 

 form the principal bulk of the body, included within a large cara- 

 pace or shield (Fig. 278, a) furnished with a long projecting spine, 

 beneath which the fin -feet are put forth; whilst the abdominal 



Fig. 278. 



Metamorphosis of Carcinus mamas : — A, first stage ; B, serond stage ; c, third stage, in which it 

 Ijegins to assume tlie adult form ; D, perfect form. 



segments, narrowed and prolonged, carry at the end a flattened 

 tail-fin, by the strokes of which upon the water, the propulsion 

 of the animal is chiefly effected. Its condition is hence compa- 



