1780, it possesses the earliest in date of the Cumacean species, unless the doubtful Gammarus esca, Fabricius, 1779, be allowed precedence. The name is evidently based on a Greek word meaning 'with an interval between the columns,' and in this Greek word the penultimate syllable is long, but the interval referred to is not really between columns, as implied by the Greek styli, but between the stilets or slender-branched uropods, to which the Latin word stili is appropriate. But to pronounce the name as Diastylis in accordance with this correction would make a hybrid of it, and it cannot therefore be recommended. This genus has the second antennæ in the male very fully developed, attaining the length of the body. The third and fourth peræopods in the female have no rudimentary exopods. The genus is widely distributed, and includes thirty species or more. Several of these are recorded by Norman, Robertson, and others, from British waters, as: Diastylis Rathkii (Kröyer), Diastylis cornuta, Boeck, Diastylis insignis, Sars, Diastylis echinatus, Spence Bate, Diastylis biplicata, Sars, Diastylis spinosa, Norman, Diastylis læris, Norman, Diastylis rugosa, Sars, Diastylis tumida (Lilljeborg), Diastylis lamellata, Norman.

Leptostylis, Sars, 1869, has the second antennæ of the male less fully developed than in the preceding genus, and has rudimentary exopods on the third and fourth peræopods in the female. The genus includes six species, of which Leptostylis producta, Norman, is British.

Diastylopsis, S. I. Smith, 1880, like Diastylis, has no rudimentary exopods on the third and fourth peræopods of the female, but it is distinguished by the unique character of having the third and fourth free segments of the peræon consolidated. To the American species, Diastylopsis Dawsoni, Smith, must be added Diastylopsis resima (Kröyer), which is well marked by the upturned nose or pseudorostrum, to which the specific name refers.

By aid of the accompanying table the student will be able to assign his specimens to their proper families, which will be found a very useful preliminary to the more difficult task of discovering the genus and the species :—

A TABLE FOR STUDENTS

313

According to the table the family can always be ascertained by the external characters, except in the case of the females of the Vaunthompsoniidæ and Leuconidæ, and there the eye is generally available, since it is absent from all the Leuconidæ, and indistinct only in one genus, Leptocūma, of the Vaunthompsoniida. The Campylaspidæ, which in the table are but slightly distinguished from the Nannastacidæ, are strongly distinguished externally both from them and all other Cumacea by the backward bulging carapace, as also by the stiliform molar process of the mandibles and internally by the single pair of liver tubes.

The genealogical relationships of the Cumacea are obscure. The second antennæ of the male resemble what is often seen in the Amphipoda. The epipod of the first maxillipeds helps the respiration, as in the Myside and cheliferous Isopods, but with the important addition of sessile branchial sacs. In the swimming branches on some of the peræopods there is a weighty resemblance to the Myside. The mouth-organs make some approaches to those of the Isopoda, and, as with them, the young are hatched before the development of the last pair of peræopods. The pleon recalls the paleozoic Phyllocarida and their existing representative Nebulia.

It will have been seen by the number of genera and species to which the name of G. O. Sars is attached that he has made a special study of this sub-order, and this sketch of it is deeply indebted to his numerous and luminous works upon the subject, to which, indeed, must be credited the clearness and accuracy with which this small but very interesting group is now known.

THE Isopoda form a vast and widely distributed army. In contrast with the distinctive uniformity of the Cumacea, they exhibit an extreme diversity of shape. The name was

KEEPING DARK

815 formed by Latreille from the Greek words loos, equal, and TOús, a foot, but, so far from the legs being all alike or equal as the name would imply, these appendages often have two or three very different developments in a single animal. There is, to be sure, a typical form of limb which prevails very widely, but the exceptional forms are numerous and remarkable. With these Latreille was unacquainted, and therefore naturally gives no clue to them in the name Isopoda, which he himself interprets as signifying 'tous les pieds simples et uniquement propres à la locomotion ou à la préhension.' In proportion to their importance in the economy of the world the Isopoda have hitherto attracted little of popular notice. They enjoy still less of popular favour. They are all of retiring habits, never needlessly courting attention, but in general clinging as closely as possible to whatever shelter or holdfast they have adopted. Amidst enormous disparities of size and strength and shape and temper, this prudent love of obscurity, the one feature of the moral character which all of them possess in common, is strong evidence that all of them must have sprung from a common origin. They have never tempted mankind to search for them as food. The services which they doubtless often render as effective scavengers are in some measure counterbalanced by the damage which some of them inflict on submarine structures and the depredations committed by others on the fruits. of the garden. Several of the species treat their fellowinhabitants of the sea with little ceremony, and make up for smallness of size by ferocity of behaviour. It is only to be hoped, as indeed it may be considered certain, that their living victims are immeasurably less sensitive to pain than ourselves.

Normally the members of this sub-order have an elongate ventrally flattened body, divided into a head of six segments under a carapace, a trunk or peræon of seven articulated segments, and a pleon usually limited to six segments. As in all the Edriophthalma, there is no appreciable ocular segment. The carapace is occasionally in coalescence with one or two of the segments of the