Sub-Class 2. Malacostraca. Order. 6. Decapoda. 223 



incapable of being straigbtened out. The carapace has a strong, 

 compressed and serrated frontal spine (rostrum). On tbe second 

 antenna is a large lamellar exopod ; tbere are long flexible flagella on 

 botb pairs of antennae, and large eyes borne upon long stalks. The 

 ambulatory appendages are thin and feeble ; the third 

 maxillipeds long and leg-like. The abdominal limbs, with 

 powerful peduncle and long laminae, are strong swimming-organs ; 

 from the inner edge of the inner ramus springs an appendage with 

 small hooks at the tip, by means of which each swimmeret is coupled 

 with its fellow, so that the two move in company. Prawns are for the 

 most part active swimmers, propelling themselves forwards through 

 the water by paddling-movements of the first five pairs of swim- 

 merets, whilst they are able to shoot backwards by powerful flexure of 

 the posterior portion of the abdomen and the caudal fin. 



Some forms differ from the rest in thafihej take up their abode in Sponges, 

 etc., and are more or less modified ia correlation with this semi-parasitism ; the 

 eyes and antennae having become smaU, and so on. 



Of the very numerous, mostly small, forms, only a few are quoted : 



1. Penxus is a genus of large Prawns reaching the size of a Oray-fish, 

 which hve only in warm seas (two species in the Mediterranean). They are 

 compressed and elongate, with small claws on the first three pahs of ambnlatoiy 

 legs. Fenxus, and some of its relatives, are distinguished from all other 

 Decapods in that they hatch as nauplii (Fig. 153) . 



2. The genus, Palxmon, is of frequent occurrence in European seas (Fig. 

 182). In this fonn only the first two pairs of ambulatoiy lunbs are chelate ; it 

 hatches, like the great majority of Prawns, as a, zoEea, and later, passes 

 through a mysis-stage. Several species of this genus are edible, as also the 

 Common Shiimp [Crangon vulgaris), which differs in many respects from Palxmon^ 

 and lives in the sand on the coasts of Great Britain. 



Sub-Order 2. Reptantia. 



The skeleton is generally thick, hard, and much calcified; the 

 animal opaque and coloured. The body is round or flat; the 

 abdomen, in some cases, very powerful and muscular, in other,s^ 

 very degenerate; the rostrum short, not compressed. The 

 exopod of the second antenna is short or absent ; the antennary 

 flagella usually feeble, the eyes small, with shorter stalks 

 than in Prawns. The second to the fifth pairs of ambulatory 

 legs are more or less powerful walking organs; the first pair 

 is, as a rule, much stronger than the rest, provided with large claws 

 and held up during locomotion ; the third maxilliped is short and not 

 leg-like. The abdominal legs are never swimming organs in the 

 perfect animal, with the exception of the sixth pair : they have, in the 

 female, the primary function of carrying the eggs, whilst in the male 

 the first two pairs serve as copulatory organs ; the three following 

 are of little importance in the males, and consequently are often 

 absent or degenerate. The sixth pair forms, in some, a well- 



