246 MANUAL OF ZOOLOGY. 



powerfully prehensile and hooked feet. The branchise are 

 attached to the first five pairs of abdominal feet. The three 

 posterior thoracic and the abdominal appendages are in the 

 form of " swimmerets/' and the tail is expanded into a powerful 

 fin. Besides the Locust Shrimps, the order includes the Glass 

 Shrimps (Erickthys) and their allies, and the Opossum Shrimps 

 (My sis}. 



ORDER II. DECAPODA. The members of this order are the 

 most highly organised of all the Crustacea, as well as being 

 those which are most familiarly known, the Lobsters, Crabs, 

 Shrimps, &c., being comprised under this head. For the most 

 part they are aquatic in their habits, and they are usually pro- 

 tected by strong resisting shells. There is always a com- 

 plicated set of " gnathites," or appendages modified for mas- 

 ticatory purposes, surrounding the mouth. The ambulatory 

 feet are made up of five pairs of legs (hence the name of the 

 order), the first pair and often some other pairs behind this 

 being " chelate," or having their extremities developed into 

 nipping-claws. The branchiae are pyramidal, and are con- 

 tained 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, furnished with three pairs of valvular 

 openings. As regards the development of the Decapods enor- 

 mous differences obtain, even amongst forms very closely 

 allied to one another. 



The Decapoda are divided into three tribes, termed respec- 

 tively the Macrura, Anomura, and Brachyura, and characterised 

 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 " Zoeae ; " * but there is 

 little metamorphosis in the common Lobster, and there is said 



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

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

 under the name of Zoea. In this stage (fig. 97) 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 carapace. 

 The "Zoea'' is separated from the "nauplius" by having a segmented 

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



