Page:VCH Warwickshire 1.djvu/213

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CRUSTACEANS They are parcelled out into three great companies, the Branchi6poda, with branchial feet, the Ostrac6da, shell-invested, which have the body completely enclosed in a pair of valves like peas in a peascod, and the Copepoda, oar-footed, which are not enclosed in valves and the feet of which are not branchial. The Branchiopoda are again subdivided into three important sections : the Phyll6poda, leaf-footed ; the Cladocera, with branching antenna? ; the Branchiura, with a name signifying that the tail is branchial. Each of these sections is represented in Mr. Bolton's list of species above quoted, although the first and third have only a single species apiece. Chirocephalus diapbanus, Prevost, belongs indeed to a subsection of the Phyllopoda which has at present no other known representative through- out England. The fairy shrimp is one of those crustaceans of which the coat is not crustaceous. Moreover it has neither enclosing valves nor extended carapace. The movements of its flexible but ill-protected body are graceful rather than rapid. Probably it is shielded from harm partly by a happy knack of lodging in unexpected places and partly by the discreet blending which nature has established between the tints of its pellucid structure and those of its environment. Its eggs, in common with those of many other Entomostraca, enjoy the wonderful power of resting in dry ground till an accession of water summons them to development. Thus after a downpour of rain this beautiful species has been known to make its appearance in such an unromantic situation as a hoof mark or a cart rut. It has long been regarded as rare, but records are accumulating which may prove it to be far from uncommon. The third section is a very small one, and its position has not always been among the Branchiopoda. Earlier authors placed its members among the parasitic Copepoda, to some of which they show a not in- considerable resemblance. This, however, may be due in great measure to similarity of habit, for all the Branchiura are parasitic on fishes or frogs, and it is some of the fish parasites among the Copepoda that they most resemble. The representative species long known in England is called Argulus foliaceus (Linn.), which may be presumed to occur in Warwickshire, whether specially recorded or not. The A. coregoni, Thorell, to which Mr. Bolton refers, is parasitic chiefly, though not ex- clusively, on Salmonidae. In this the great shield covers all the four pairs of swimming feet, whereas in A, foliaceus the fourth pair is left exposed. Both alike have a pair of large lateral eyes and a small trilobed median eye. In this genus the large sucking disks into which the maxillae at a certain stage of development are metamorphosed betray the parasitic character of the animals. Yet they can exist for days, or even weeks, apart from their hosts. For leading a life of independent activity they have first to be well gorged, and to this end, it has been observed, nature has provided them with ramified ccecal appendages in the gastric depart- ment. Dr. Baird has quoted Jurine's observation that fishes seemed to be afraid of these little vampires, and would speedily reject them if acci- dentally swallowed. This may be true in general, but the late Professor 175