138 



I.WEETEBEATE ANIMALS. 



posed of a convex upper plate {t) and a flatter ventral arch (s), and 

 each may support a single pair of appendages. Typically, each 

 appendage in a Crustacean consists of a short basal portion or " pro- 



topodite " (fig. 86, a), which 

 gives origin to an outer and an 

 inner branch or division, known 

 respectively as the " exopodite " 

 (6) and the " endopodite " (c). 

 Very commonly one of the two 

 divisions of the appendage may 

 be wanting ; or in some cases 

 both are absent, and the ap- 

 pendage may be reduced to the 

 protopodite only. A certain 

 number of the appendages of 

 the anterior segments are usu- 

 ally converted into masticating 

 organs, some of which may be 

 proper biting jaws (mandibles 

 and maxillae), while others are 

 merely limbs so far modified 

 as to act as subsidiary jaws 

 (" foot- jaws '' or maxillipedes). 

 The last segment of the body — known as the " telson " — carries no 

 appendages, and is often regarded as not being properly a segment, 

 but an unpaired appendage. 



The Crustacea form an extremely large group of animals divided 

 into many orders, and it will be sufficient to merely notice many of 

 these in the briefest manner. It will also be as well to commence 

 with the consideration of the higher sections of the class first. 



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Fig. ^Q. — The third abdominal segment of 

 the Lobster. ( Dorsal arch ; s Ventral 

 arch of the segment ; a Protopodite ; 6 

 Exopodite ; c Endopodite. 



Order Decapoda. 



The Cnistrieea included in this order derive their name from the 

 fact that they all possess five pairs of legs (Gr. deka, ten ; podes, 

 feet). They belong to a large section known as the " stalk-eyed " 

 Crustaceans [Podophthalmuta), from the fact that the eyes are sup- 

 ported by long, movable st:dks. They all further have the cephalo- 

 thorax covered by a great shield or " carapace," under which the 

 gills are carried. They include the Lobsters, Shrimps, Cray-fish, 

 Crabs, Hermit-crabs, and other forms, and are the most highly 

 organised and most familiar of the whole class of the Crustacea. 

 They are divided into three very well marked groups or tribes, all 

 of which can be exemplified by familiar types. 



