CRUSTACEANS the two crustaceans already discussed the advantage of being extremely abun- dant. Any little weedy brook will almost certainly supply it. In such a streamlet it occurs not far from the railway station of Berrington and Eye, and in a duck-pond of the same district. It differs from the other two species in that, though common in occurrence, in character it is rather fincommon. It cannot be regarded as an eminently representative species. There are, however, a goodly number of the Isopoda of which the same might be said, and notwithstanding its peculiarities Asellus shows some of the commoner characters with great distinctness. It has the dorso-ventrally flattened body which gives the Isopoda in general walking power so much superior to that of the laterally compressed Amphipoda. Its seven pairs of legs have a fair amount of that uniformity which suggested the title of the order, a title far from being appropriate in all the sub-divisions. Minute comparison of the cephalic compartment or head of Gammarus with that of Asellus will show that from the eyes to the first maxillipeds the amphipod and the isopod are in rather close agreement. Also the middle body or peraeon with seven articulated segments, each carrying a pair of similarly jointed limbs, continues the resemblance. But a critical inspection of this part reveals a divergence between the two orders of no trifling importance. The amphipod has at the base of the second gnathopods and of the first four pairs of peraeopods freely- hanging vesicles, or branchial sacs, answering to the more complicated and more numerous branchiae of the crayfish. But these are entirely wanting in the isopod. There the respiratory function is transferred to the pleopods. To correspond with this difference the lateral openings of the heart, instead of being placed in the front part of the peraeon, are withdrawn to the rear. Among the more unusual characters of the genus Asellus it may be noticed that ' the basal joint of each maxilliped possesses in the ovigerous female a rather large plate, bearing a number of bristles at the end and directed back- wards ; it has been mentioned and well drawn by Sars ; its function is certainly to produce a current of water in the marsupium.' ^* The marsupium or incubatory pouch is formed by four pairs of oval plates or membranes springing from the base of the first and second gnathopods and the first two pairs of peraeopods. The close overlapping of these oostegites or egg-covers forms a compact oval reservoir beneath the front part of the peraeon. ^° The pleon or tail-part of Asellus aquaticus, as it is functionally differentiated from that of Gammarus pulex, so also superficially it is very unlike. Not only by its flatness and relative shortness is it strongly distinguished, but instead of seven distinct segments here there seems to be only one. The fact is that the seg- ments, apart from a rudimentary first one, are consolidated into a caudal carapace much after the fashion of the cephalic carapace at the other extremity. The long pair of apical appendages are the uropods, proper to the sixth pleon segment, the small projection of the caudal shield between them no doubt representing an unarticulated telson. The earlier segments must be inferred from their pleopods, of which the first two pairs are much modified in the male and the second pair are wanting in the female. The Isopoda terrestria are sometimes held up to reprobation as pests of the garden. It is very questionable whether they ever do any serious amount " H. J. Hansen, Proc. Zool. Soc. Lond. (1904), ii, 305. "G. O. Sars, Crustacis d'eau douce de Norvege (1867), p. 113. 117
