head and other parts of the whale while the monster is still alive. On the other hand, whales and seals, and fishes. large and small, swallow down the Crustacea in a truly wholesale manner, and so prevent these prodigiously prolific animals from producing a complete block in the cooler parts of the ocean. Independently of this interesting exchange of courtesies, which consists in alternately eating and being eaten, there is another kind of association between crustaceans and other animals, known as commensalism. In this the one creature lives, not at the expense of the other, but merely in companionship with it. Thus, on the common starfish there is found a threadlike minute species of the Caprellida. Though the starfish is very frequently to be met with on the shores, its companion Pariambus typicus (Kröyer) is only seen on dredged specimens, so that apparently this tiny animal. has the sense to disengage itself when its host is being driven into an unsuitable or dangerous position. Some of the Amphipoda Hyperidea are very frequently to be found upon jelly-fishes. One of the Gammaridea, Isca Montagui, Milne-Edwards, appears never to have been found except upon the Spinous Spider Crab, Maia squinado, for clinging to which its feet, with their serrate widened extremities, are peculiarly adapted. A French zoologist, M. Edouard Chevreux, some five or six years ago, was searching this interesting crab for its already well-known commensal, when to his surprise, among the algae and hydrozoa, with which the carapace is usually decked, he found not only the species he was in search of, but no less than twentytwo other species of Amphipoda into the bargain. Maia squinado is not found far to the northward. On the other hand a very distinct crab, but with some external resemblance to it, Lithodes maia, is not found far to the southward. Such facts of distribution are often of scientific importance. For instance, with regard to the comparatively narrow strip of land which separates North from South America, the geologist will desire to know how far the crustacean fauna of the sea on one side of the

'Formerly Podalirius typicus.

isthmus corresponds with that on the other. A close agreement would dispose him to consider that not so very long ago, in the large measurements of geological time, the Atlantic and Pacific may have been connected by a natural canal. From great divergence, such as is known. to exist in the molluscan fauna, he would infer that the passage existed, if ever, only at a very remote period. In the large freshwater lakes of Southern Sweden, it was discovered some thirty years back, that a remarkable marine fauna existed, and the inference ingeniously drawn from a review of all the connected facts has been, that these sheets of water were at one time part of the sea, but have been cut off from it by the gradual elevation of the land. Upon this supposition, while the water was gradually losing its saltness, its marine inhabitants, with equal steps, were becoming habituated to a freshwater existence. But it must not be forgotten that the transfer of marine animals to brackish and fresh waters may take place by various modes of migration quite independently of geological changes. It has been noticed as curious, that shells, insects, and plants, inhabiting fresh water, are of comparatively few species, but those few very widely distributed. Mr. Belt ingeniously remarks that, in the oscillations of sea and land, the oceanic and continental domains, though shifting, are continuous, whereas every freshwater area is liable to be again and again completely overwhelmed. By this means the freshwater species of narrow range may be entirely destroyed, and only families of wide distribution will survive. The application of this theory to the Crustacea is worthy of study, but the facts which it is designed to explain do not embrace the whole of the globe. The Isopod Asellus aquaticus and the Amphipod Gammarus pulex are obvious. instances of freshwater species with an enormous range. Yet from the fresh waters of the Malay archipelago the Asellidæ and Gammaridæ are said to be entirely wanting. On the other hand, Professor Max Weber has recently observed that, while Europe can show but seven species of freshwater decapods, the Indian archipelago can boast of more than eighty.

CHAPTER III

MAGNITUDE

IN zoology, size attracts attention in comparison with two standards. Man contrasts the bulk of an animal either with the average in his own order or with that of the group to which the particular animal belongs. An elephant is a huge beast, not in competition with a whale, but with a human being. A hornet less than a man's little finger is a monster beside a house-fly or a gnat. As in all classes the majority of individuals conform pretty closely to the average magnitude, the mind becomes trained to regard the exceptional extremes with wonder, often not unmingled with admiration when the mass is not smaller but greater than common. Among the Crustacea there are forms, not indeed surpassing all others in diminutiveness, but at any rate so exceedingly small that the sharpest eyes could perhaps never have found out that they were crustaceans without the aid of the microscope. Here it is that the philosophical naturalist sometimes finds chief reason to marvel, in perceiving the whole machinery of life, enabling active locomotion, nutrition, and reproduction, with senses, a power of choice, and the capacity for feeling pleasure and pain, all packed away as neatly and conveniently as possible in so extraordinarily small a casket. For preliminary studies creatures of more considerable compass hold an obvious advantage, and with most observers it is rather the giants of an order than the dwarfs that are deemed especially remarkable and worthy of the notice which they are fitted readily to engage. Some of the old writers probably understood

Pl. i

ASTACI MARINI, QUEM HVMER vocant Germani, ex defcriptione Septentrionalium regionum Olai Magni,effigies. Ingentem effe fcribit, (inter Orchades & Hebrides infulas,) &tam validum vt hominem natantem chelis apprehenfum fuffocet. Sed non probo, quod pedes omnes bifculcos pinxit: caudam tabellis tam multis con

ftruxit, &.