136 



NATURAL HISTORY. 



Fig. 123. — Nebulia, enlarged. 



certain other features ' superadded 



some small forms, one of which is illustrated in our cut. These are small, 

 inconspicuous forms of but little interest to any but the professional natu- 

 ralist, and are introduced here 

 merely as an illustration of the 

 fact that in nature there are no 

 sharp transitions. As will be seen 

 this animal bears a general resem- 

 blance to the fairy-shrimp figured 

 on a preceding page, but there are 

 Thus the anterior part of the body is 

 enveloped in a shell, or carapax, as it is called, and this carapax, though 

 occurring in other groups, is very characteristic of the decapods. 



The term decapod means ten-footed, and almost all the members of the 

 group are characterized by the possession of ten feet adapted for prehen- 

 sile or locomotor purposes. They have besides twenty segments in the 

 body, and the eyes are seated upon movable stalks which are short in 

 most forms, but occasionally reach a considerable length. In size, although 

 they embrace many small forms, they also include the largest of the Crus- 

 tacea, or indeed of the Arthropods. A spider-crab which occurs in Japan 

 has a proportionally small body, but its legs stretch out to a great extent, 

 and specimens are found which, measured from the tip of one leg to the 

 tip of the opposite member, will exceed twenty feet. None of the other 

 forms begin to reach this size, but there are several which considerably 

 exceed it in the size of the body. 



Lowest of the decapods are the prawns and shrimps, which embrace a 

 large number of species of both salt and fresh-water forms. Most of them 

 are small, but one of the tropical species reaches a length of twenty inches 

 from the end of the abdomen to the tips of the out-stretched claws. One 



Fig. 124. — West Indian prawn (Palsemon jamaicensis). 



of these species from Mexico and Central America is represented one-fourth 

 oi the natural size in the cut. It occurs in great abundance in the fresh 

 waters of that locality, and is used as food. In the northern seas occurs 

 the true shrimp (Crangon vulgaris), which is found on both coasts of 



