HIGH SCHOOL ZOOLOGY. 

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197 



Fi?. 122.— Diairrani of circulation of Crayfish— a, heart in pericardium ; 6, c, d, an- 

 terior, posterior, and ventral arteries. 



lobster, as well as most other Crustacea, the young are freed 

 from the egg when thej have attained three pairs of legs 

 (ISTauplius-phase) ; they only arrive at the adult form after a 

 series of moults, and there is generally a complicated meta- 

 morphosis. 



12. If we except a species of prawn {Palcemonetes) and another of 

 Opossum-shrimp (3I//sl.s) found in the Upper Lakes, the Podophthalmata 

 are exclusively marine forms, including on the one hand the various 

 kinds of shrimps and prawns, which resemble the crayfish in the long 

 tail {Macrura), and on the other, the crabs (Brachyura), where the short 

 tail is tucked up under the cephalothorax. An intermediate group is 

 formed by the hermit-crabs (Paguridce), in which the cuticle of the 

 tail-segments never becomes calcified, and the creatures resort to empty 

 univalve shells for protection. An allied East Indian genus, the cocoa- 

 nut crab (Birrjus latro), lives in holes in the earth, and, instead of 

 depending on its gills for respiration, uses the wall of the gill-cavity as 

 a lung. This is an instance of what is termed "change of function," 

 a principle which must be borne in mind, in comparing the structure of 

 animals which are nearly allied in form, but different in habits. 



1 3. Two other orders of Crustacea, which resemble the cray- 

 fish in the number of the segments and the appendages, have 

 fresh-water representatives which are very common, although 

 the majority of both are marine. These are tlie Isopoda and 

 the Amphipoda ; but in both, only one of the eight pairs of 

 thoracic appendages is turned forwards toward the mouth. 

 The Isopods have the body depressed, while in the Amphipods 

 it is compressed. A familiar example of the former is the 

 water-slater, Asellus communis. (Fig. 123). It will be observed 



