MARINE ARTHBOPODS 271 



abundant in the rock pools of our coasts, particularly in the south- 

 west, but their bodies being often so transparent as to be almost 

 invisible, they are consequently easily overlooked. Their general 

 appearance may be gathered from our illustration of Myais 

 chamceleon, which is probably the most common species inhabiting 

 our coast. 



The highest crustaceans the Decapods are divided into two 

 sub-orders the Macrura, or Great-tailed, in eluding lobsters, shrimps, 

 &c. ; and the BracTiyura (Short-tailed), containing the crabs; but 

 the number of British species is so large that it is impossible to 

 give, in our limited space, a detailed description of all the commonest 

 even. All we can do is to note a few of the more interesting 

 features of certain species, to introduce such illustrations as will 

 enable the young naturalist to identify a number of the commoner 



Fro. 202. THE OPOSSUM SHEIMP (Mysis chamceleon) 



ones, and to give the general characteristics of the main divisions 

 so that the student may be able to classify his specimens 

 intelligently. 



In the Macrura, as with other divisions of the crustaceans, we 

 meet with very interesting modifications of the appendages, adapted 

 to quite a variety of uses ; and if the reader is unacquainted with 

 these adaptations of structure to habit he cannot do better than 

 secure a lobster or crayfish for study. It will be observed that the 

 body may be divided into two main portions the cephalothorax, 

 consisting of head and thorax combined, and the abdomen. The 

 former is composed of fourteen segments, so thoroughly fused 

 together that they are denoted only by the fourteen pairs of 

 appendages to which they give attachment, while the calcified akin 

 forms one continuous shield surrounding the whole. The abdomen, 

 on the other hand, consists of six distinct segments, each of which 



