ANNULOSA: CRUSTACEA. 307 



late" or having their extremities developed into nipping-claws. 

 The branchiae are pyramidal, and are contained in cavities at the 

 side of the thorax. The carapace is large, covering the head and 

 thorax, and the anterior part of the abdomen. The heart of the 

 Decapoda is in the form of a more or less quadrate sac, fur- 

 nished with three pairs of valvular openings. As regards the 

 development of the Decapods enormous differences obtain, 

 even amongst forms very closely allied to one another. 



The Decapoda are divided into three tribes, termed respec- 

 tively the Macrura, Anonmra, and Brachyura, and charac- 

 terised by the nature of the abdomen. 



TRIBE A. MACRURA. The " long-tailed " Decapods included 

 in this tribe are distinguished by the possession of a well- 

 developed abdomen, often longer than the cephalothorax, the 

 posterior extremity of which forms a powerful natatory organ 

 or caudal fin. As regards the development of the Macrura, 

 most appear at first in the form of "Zoese;"* but there is 

 little metamorphosis in the common Lobster, and there is said 

 to be none in the Cray-fish (Astacus fluviatilis}. Fritz Miiller, 

 again, has shown that the primitive form of one of the Shrimps 

 (Peneus] is that of a "Nauplius." Lastly, the young of the 

 Spiny Lobster (Palinurus vulgaris) are transparent Phyllosomce, 

 resembling Stomapods in appearance. This section comprises 

 the Lobster, Cray-fish, Shrimp, Prawn, &c., of which the Lob- 

 ster may be taken as the type. 



In the Lobster (figs. 136, 137), as also in the Cray-fish 

 (fig. 157), the somites of the head and thorax are amalgamated 

 into a single mass, the " cephalothorax," covered by a carapace 

 or shield, which is developed from " the lateral or epimeral 

 elements of the fourth cephalic ring, which meet along the 

 back, and give way preparatory to the moult. The tergal 

 elements of the thoracic rings are not developed in either 

 Crabs or Lobsters ; when these rings are exposed by lifting up 

 the cephalothoracic shield, the epimeral parts alone are seen, 



* The young Decapod, in most cases, leaves the egg in a larval form so 

 different to the adult that it was originally desciibed as a distinct animal 

 under the name of Zoea. In this stage (fig. 160) the thoracic segments 

 with the five pairs of legs proper to the adult are either wanting or are 

 quite rudimentary. The abdomen and tail are without appendages, and 

 the latter is composed of a single piece. The foot-jaws are in the form of 

 natatory forked feet, and the mandible has no palp. Lastly, there are 

 no branchiae, and respiration is carried on by the lateral parts of the cara- 

 pace. The "Zoea" is separated from the "Nauplius" by having a seg- 

 mented body, large paired eyes (sometimes with a median eye), and a 

 carapace. The form proper to the adult is not attained until after several 

 moults, constituting a genuine metamorphosis, though one which is effected 

 by very gradual stages. 



