5532 Crustacea. 



ground. This is the most natatory of all our crabs, which might be expected to be the 

 case, from the oar-shaped character of all the legs. The drift-net fishery allows the 

 nets to drift with the tide as a floating wall, and these crabs are almost always captured 

 on the eastern side, showing therefore that they are arrested in a westerly migration. 

 I regret that we have no spring fishery, that it might be ascertained whether there is 

 a return migration. All the specimens that I have seen are full-grown adults, and 

 what is still more worthy of notice, all are males ; out of several thousands I did not 

 discover a single female, so that this species not only migrates, but it is the male alone 

 that does so, so far as my observations go. If the females and males migrated in 

 separate flocks, some of both sexes would be most probably taken, where the boats are 

 so numerous ; but every season produces the same result ; the males alone being taken. 

 But, from the size of the specimens taken, they are no doubt adults, and are much larger 

 than one season would permit. It may therefore become a subject for enquiry, whether 

 all the males, young as well as old, migrate, or whether the migration is confined to the 

 adults. The females apparently remain in the shallow waters of the channel, and the 

 males must in some way or other return to them in the early spring. Probably, how- 

 ever, as in other species, impregnation takes place one summer, while the ova are shed 

 the next. This point, with many others equally interesting, I have ascertained by 

 direct experiments, and in the lobster, while one set of ova are being shed others are 

 rapidly developing themselves in the ovaries, to be impregnated after the shedding has 

 been performed. During the period of what may be termed gestation, there is no 

 exuviation, but generally within thirty hours after the ova have been all cast the shell 

 is thrown off, and then most commonly impregnation takes place ; but if the female be 

 not impregnated, exuviation occurs very frequently during the spring, summer and 

 autumn, varying with the age of the specimen. During the years 1855 and 1856, 

 I kept several specimens alive in my tanks, for the purpose of watching their habits: 

 for several days they kept quiet at the bottom, rarely seeking the shelter -either of the 

 stones or sea-weed, and even when stirred they seemed so sluggish that they rarely 

 did more than just move out of the way. On placing a little sand at the bottom, they 

 soon made an excavation, and sank into it, partly covered ; towards the evening, how- 

 ever, or if the day was dull, they became more active, crawling rapidly from spot to 

 spot. Sometimes they would creep on the stones and launch themselves off the edges, 

 and at others they would elevate their bodies either against the glass or a stone, and 

 rapidly vibrate the posterior claws ; by this they rapidly rose to the surface, and in 

 rising all the claws were frequently in motion ; but in their progress, whether round the 

 tank or obliquely, either in ascent or descent, the claws anterior in progress were the 

 only ones used, the posterior ones being merely stretched out : the claws most effectively 

 employed were the posterior two, and hence in progression the posterior part of the 

 body became more elevated than the front. They swam with ease and rapidity, and 

 would continue in motion for hours in a subdued light. Like most other Crustacea, 

 the chief part of their movements take place at night: I have occasionally, by irri- 

 tating them, forced them into activity by daylight, but their efforts soon cease, and 

 they again sink to the bottom. They are very voracious, as well as active, seizing on 

 the pollack, cod, and even mackerel, but they do not refuse dead food, as they de- 

 voured very eagerly bits of whiting which were given them from time to time. 

 Although this species is so very active in its habits, and apparently voracious in its 

 appetite, and is the most natatory of all British species, it would seem that they occa- 

 sionally may be said almost to hvbernate, as in some specimens dredged off Scilly 



