THE SESSILE-EYED CRUSTACEANS

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CHAPTER XX

ORDER II.-EDRIOPHTHALMA

In this order there is generally a pair of compound eyes, which are sessile; they may be prominent, but are never movably stalked; they may be absent, or placed so closely together as to form apparently, but not really, a single eye. On the other hand, the visual elements may be variously distributed, so as to form compound eyes from one to four in number, or simple eyes that are not limited to four. The last seven segments of the trunk are generally not, and the last four are never, included in the carapace.

Three sub-orders are comprised in this order-the Cumacea, Isopoda, and Amphipoda.

Sub-order 1.-Cumacea.

As with the Squillidæ, so with the Cumacea, nothing is to be found about them in the writers of antiquity, but about the latter sub-order even the sixteenth century knew no more than its predecessors. Not a single species was recorded before the year 1779, and for sixty years after that date science contented itself with half a dozen species, some of which were so obscurely or imperfectly described that they are still doubtful. Since 1841, by the exertions of a few naturalists, chiefly Scandinavian and Anglo-Saxon, the characters of the group have been investigated with great thoroughness. The distribution extends to all oceans, and from tide-marks down to very great depths. By rapidly repeated discoveries the known forms have now become numerous, and are classified in

eight families, together containing twenty-five or twentysix well-defined genera, and about a hundred and eighteen satisfactorily determined species. In certain localities the extreme abundance of some of these supplies a welcome food to shoals of herring. In size and number of individuals the Arctic species appear to excel all others. It is probable that many forms still remain to be discovered, since up to 1876 a single species obscurely described from the Black Sea was the sole representative of the sub-order in the area of the Mediterranean, where in the winter of 1876 Professor Sars found no less than twenty-three species, of which fourteen were new to science. Before 1859 none were known from the waters of the Clyde, and in these Mr. David Robertson, of Cumbrae, has since found fifteen species.

Among the curiosities of scientific literature are the disputes which have occurred actually within the last fifty years, as to whether the Cumacea did or did not possess organs of vision, and as to whether they were or were not merely larval forms. The well-known and diligent observer, Colonel Montagu, at the beginning of the century found in South Devon a species which in fact possesses sight, but not unnaturally Montagu did not attempt to discover the creature's eye, because he was under the impression, though an erroneous one, that his solitary specimen had lost its head. Kröyer, many years later, happening to meet with various species which are in truth blind, formed the opinion that all the species were so, and that eyes had been attributed to some of them under an illusion. Harry Goodsir, a Scotch naturalist, in 1843 published the remarkable statement that the eyes are 'pedunculated but sessile.' He lost his life not long afterwards in Franklin's Arctic expedition, and left his opinion to be for many years doubted, denied, or supported, without its being in his power to explain that what he obviously intended to print was that the eyes were not pedunculated but sessile. It is probable that Kröyer was completely mystified by the misprint. 'Goodsir,' he says, thought that eyes must be found in the

6

CHARACTERS OF THE CUMACEA

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Some years

creatures, and he therefore found them.' earlier Milne-Edwards had indeed described a species as having two eyes, which almost certainly has but one; so easily are even the ablest naturalists misled into taking for granted that what is customary will prevail. A very few of the Cumacea have two eyes. The majority have a single median eye, but occasionally the elements of this compound organ are so arranged as to have the appearance of several distinctly separated ocelli.

The Cumacea are said to leave the egg as maggotshaped Nauplii. From the maternal pouch they issue almost in the adult form, only being as yet without the last pair of trunk-legs or peræopods, a deficiency which in two species, Campylaspis nodulosa, Sars, and Leptostylis manca, Sars, is perhaps persistent. The notion that these creatures, which are in truth born almost fully fledged, were only larval forms, was early disproved by Goodsir and Kröyer, but was nevertheless still upheld for several years by three of the most distinguished naturalists of the century.

In no group of the Malacostraca is the general form more characteristic than here. This is principally due to two unvarying features, the narrowness of the pleon and the prominence of an elongate pair of uropods at its extremity. A distinct telson may or may not be present; of the six preceding segments none is ever wanting, and of these the fifth, without any trustworthy exception, is the longest. The front part of the body is always more bulky than the pleon, and sometimes enormously so. The integument is often stoutly crustaceous, more ready to break than to bend.

Normally there are five segments of the trunk distinct behind the carapace, carrying respectively the five pairs of peræopods. Occasionally, however, the first of these five. segments is absent or rudimentary, and in one genus the third and fourth segments are coalesced. Sometimes the carapace overarches one or more of the free segments. More or less completely sheltered beneath the carapace lie all the organs from the eyes to the third maxillipeds. There

are no chelate limbs to which the terms chelipeds or gnathopods would be applicable.

The adult males are very strikingly distinguished from the females in more than one particular. Thus, the females have the lower antennæ of insignificant size, while in the males they are developed to a great length, though they are not always very apparent, as they are often securely tucked away along the sides of the body, an arrangement which seems to imply that they are of exceptional value, and possibly also only of exceptional use. Again, the females never have pleopods, while the males have them, except in a single genus, throughout six of the eight families. Moreover, in all the families but one the males have welldeveloped exopods or swimming-branches attached to all the first four pairs of peræopods, whereas the females never have them on more than three pairs, and sometimes only on one or two. These disparities are connected with the habits of the animals. For at night-time it is found that the surface-net in suitable localities will secure the males of certain species in abundance, not intermingled with any females. The latter sex, on the other hand, is generally found to preponderate when their settlements in the sand and mud are invaded. The females are evidently of a less roving disposition than their mates. Yet their movements are not destitute of considerable liveliness and energy. To compensate them, too, for their inferior swimming apparatus, they very commonly have the carapace ornamented with various spines and tubercles, which may be supposed to form a sort of defensive armour against some of their foes. The more active males, on the contrary, seem to find their advantage in having the carapace smooth. To the student this last distinction is not a little embarrassing, for much smaller differences than those which here mark the external appearance of the two sexes have frequently been allowed specific value.

The upper antennæ of the Cumacea are always small, with two diminutive flagella, of which one is sometimes evanescent. The upper lip is a single lobe between the bases of the lower antennæ. The lower lip forms two

MANDIBLES AND MAXILLE

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lobes with finely ciliated edges. Between the lips the hard and strong mandibles make their dentate cutting edges meet. They have no 'palp.' The left mandible, and rarely also the right, has a secondary plate on the inner side. The molar tubercle is generally a long and strong process at right angles to the body of the mandible, with a denticulate crown; but in one family, the Campylaspidæ, the process is thin and stiliform. Between the cutting edge and the molar there is usually a row of spines. It is, however, distinctive of one family, the Leuconidæ, that this spine-row is absent, being represented only by a couple of setæ. The first maxillæ may be roughly described as consisting of an inner and outer plate, each tipped with spines, and of a backward directed 'palp.' The inner plate is an expansion of the first joint, and the outer plate an expansion of the third, the small second joint between them being very little conspicuous. Close to the base of the outer plate on its outer side is placed the palp,' which may be the fourth joint of the endopod turned back to serve a particular purpose. It is very movably articulated, and ends in one or two long setæ. Its function is reasonably supposed to be that of clearing the branchial sacs from any obstructive particles which might be brought in along with the water introduced by the action of the breathing apparatus. In the genus Paralamprops, however, this service must be dispensed with, for there the 'palp' does not exist.

The second maxillæ are analysed by Hansen into four joints, of which the second is very large, having a row of setæ on the inner margin and a small terminal plate tipped with setæ. The third and fourth joints have also each a similar small terminal plate, and to the outer rim of the third joint is attached a thin plate with curved margin, which is explained to be morphologically a weaker development of the fan as it occurs in the Myside.

Upon the next pair of appendages, the first maxillipeds, the interest of the Cumacean group is above all concentrated, for these present features that are unique among the Crustacea. The endopod or limb-like portion of the