266 THE SEA SHORE 



locomotion, but are not adapted for swimming ; and two pairs of 

 antennae, one of which assists in locomotion. The mouth is pro- 

 vided with organs of mastication, the branchiae are attached to the 

 hind jaws, and the animals have but one eye. Some of these 

 crustaceans inhabit deep water only, while others live in sand 

 between the tide-marks ; but several species, belonging chiefly to 

 the genus Cythere, abound in rock pools, where they may be 

 readily obtained by scraping the confervas and corallines with a 

 small muslin net. 



The branchiopods are free swimmers, and are protected by a 

 buckler-like envelope. Most of them are inhabitants of fresh water, 

 and are popularly known as water fleas. We have figured one 

 marine species, belonging to the genus Evadne, which has a colour- 

 less body, and a single conspicuous black 

 eye, and is interesting as being the food 

 of the herring. 



The four orders of crustaceans that 

 have been briefly described belong to the 

 division Entomostraca, which signifies 

 ' shelled insects.' This term is not a 

 FIG. 198. Evadne happy one when judged from the stand- 

 point of our present knowledge of animal 



life, but it must be remembered that, at the time it was applied 

 (1785), spiders and crustaceans were all included in the same class 

 as the insects ; and this is hardly surprising when we observe the 

 close relationship of these animals, as shown in their segmented 

 bodies and jointed appendages ; for, as we have already shown, the 

 lowly organised parasitic crustaceans which, in the adult state, lose 

 most of their appendages and cease to be distinctly segmented, are 

 more or less insect-like in their larval and free-swimming stage. 



All the other crustaceans are included under the term Mala- 

 costraca, or soft shelled, since, although many of them are protected 

 by an exo-skeleton that is hardened by the deposit of carbonate 

 of lime, yet, generally speaking, their coverings are* softer than 

 those of the molluscs ; and therefore the term Malacostraca was 

 originally applied by Aristotle in order to distinguish them from 

 the animals that are covered by harder and thicker shells. 



This division of the crustaceans contains wood lice, sand- 

 hoppers, lobsters, shrimps, crabs, &c., and consists of two main 

 groups the Sessile-eyed (Edriophthalmata) and the Stalk-eyed 

 (Podophthalmata) crustaceans. 



