CRUSTACEANS cunning. It is therefore only with some reserve that G. pulex can be called ' the freshwater shrimp.' Shrimps, in the more familiar accepta- tion of the term, are all stalk-eyed. Furthermore our common shrimp and common prawn are phyllobranchiate, that is to say they have under the carapace a series of breathing organs composed of two rows of branchial leaflets. On the other hand in the Amphipoda the branchia? or gills are not under the carapace, and are as a rule undivided, each consisting of a single vesicle. There are true freshwater shrimps and prawns of the same general character as the marine species to be found in many places, though they do not happen to occur in Warwickshire. Hence Amphipoda are spoken of as shrimps only because popular neglect in the past has left them without any suitable vernacular appella- tion. Apart from the want of pedunculate eyes however they have as many jointed appendages as the ordinary eatable shrimp. The head as usual carries two pairs of antennas. These are followed by four pairs of jaws, known as mandibles, first and second maxilla? and maxillipeds. With these the carapace or cephalothorax comes to an end, and is succeeded by the middle body made up of seven separate segments carrying seven pairs of legs, after which comes the normally jointed pleon with its six pairs of appendages that have various functions of swimming, springing or promoting a circulation of the surrounding water. In a shrimp or lobster, on the other hand, the carapace includes both head and middle body, carrying the two pairs of antenna? and six pairs of jaws instead of four, but only five pairs of legs instead of seven, the pleon both here and elsewhere remaining uncovered. Among all the more or less striking differences however, the total number of appendages between the eyes and the pleon it will be seen is precisely the same in the decapod or ten-footed macruran and in the tetradecapod or fourteen-footed amphipod. Not only is the number the same, but the appendages themselves are evidently equivalent, homologous, pair for pair, though in the case of some of them science has been pleased to vary their names and nature has been pleased to vary their functions. Upon ' the water woodlouse, Asellus vu/garis,' somewhat similar observations may be made. The name to be preferred for it, as older than Latreille's A. vutgaris, is A. aquations (Linn.), and for this Latin term ' water woodlouse ' would be as fair an English equivalent as could be given. In our inland counties it might even deserve to be distin- guished as the water woodlouse, because in those counties the order Isopoda to which it belongs has no other freshwater representative. Nevertheless the title woodlouse is not well fitted to animals that live only in the water, and besides it belongs by right to a large terrestrial subdivision of the order. The Isopoda are sessile-eyed malacostracans like the Amphipoda, and have almost the same arrangement of append- ages. They also have the middle body uncovered by the carapace. Still between the two orders the differences are many and important. In the genuine isopods the heart is in the hinder half of the trunk instead of being as in the amphipods in its front half, and in place of 173
Page:VCH Warwickshire 1.djvu/211
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