OPTICAL INSTRUMENTS

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tion. It is unusual to think of a creature's eye as one of its limbs, for only by a figure of speech do we describe a person as grasping the whole situation at a glance. Nevertheless, there are very few inclined to dispute that the eyes of the Podophthalma have really been developed upon appendages by nature equivalent to the rest of the series. Any one acquainted only with the extremely short-stalked eyes of some of the crabs might be excused for thinking this view extravagant, but its improbability is lessened when we observe the long-stalked eyes of the Angular Crab, Gonoplax rhomboides (Linn.), or those of the macruran Leucifer Reynaudii, Milne-Edwards, or those, again, of Eretmocaris longicaulis (see Plate XIII.), a shrimp in which the eye is projected on a support actually longer than the animal. Moreover, the ocular appendage, besides being articulated to the head, is itself composed of two or three articulations. In the fast-running Ocypode cursor (Linn.), the peduncle is extended beyond the cornea of the eye, and terminates like an antenna in a pencil of long hairs. There is one instance on record in which the eye of a kind of lobster, Panulirus penicillatus (Olivier), has been observed to develop a jointed antenna-like lash, while the companion eye remained normal. This evidence is parallel to that on which a botanist infers that the petals of a flower are by origin modified leaves when he sees them occasionally assuming the form of the unmodified leaf.

2. The second segment carries the first pair of antennæ, sometimes called the inner or upper, or, without epithet, the antennules. In the Malacostraca normally these appendages consist each of a three-jointed peduncle and two flagella or lashes, composed of many joints or few, the inner or secondary flagellum being not unfrequently absent altogether or rudimentary. In some instances, and in the Amphipoda Caprellidea and in the Entomostracan Copepoda not as an exception but as the rule, the first antennæ are larger than the second, from which it results that the diminutive name antennules is rather convenient than appropriate. The superior size, however, is no indication of higher rank, but rather the

reverse. These organs are sometimes most dwindled in families which can claim a decided precedence over others in which these appendages are well developed. Thus they are short in the crabs, but long in the lobsters and shrimps, and short in the normal Isopods, but long or large as a rule. in the Amphipoda, and within the Amphipoda they are short in the Orchestidæ, a family that claims superiority by its tendency to terrestrial habits.

Those who have made themselves acquainted with Professor Huxley's volume, 'The Crayfish,' in the International Scientific Series, will be aware that in describing a crustacean appendage he names the first two joints the protopodite, which bears at its extremity on the inner side the endopodite, and on the outer side the exopodite. For these terms the shortened forms exopod and endopod will here be preferred-exopod for exopodite, endopod for endopodite and protopodite combined and peduncle will be used for a variable number of basal joints. In the first antennæ the peduncle consists, as already stated, of three joints, and by this circumstance the rule which widely prevails elsewhere that the so-called protopodite ends with the second joint of an appendage is broken without any obvious cause. Moreover, that which by its function and in general by its superior size appears to be the main branch is here the outer one, and not as usual the inner. It is conceivable that the exopod is wanting, that the main branch or principal flagellum is the true endopod, and that the secondary flagellum is an independent outgrowth. For the reasons mentioned, and some others, Dr. J. E. V. Boas considers that the first antennæ are not homologous with the following limbs, but that both they and the stalked eyes ought to be regarded as limb-like sense organs. That besides being organs of touch, they are frequently organs of other senses, seems to be beyond doubt. In the Macrura at large the first joint contains an auditory apparatus. Sometimes the cavity is provided with a well-formed otolith or ear-stone. In the lobster and crayfish, Mr. Spence Bate says, 'the perforation is long, narrow, and slit-like, the aperture being scarcely

AN ACOUSTIC TAIL

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appreciable, and opens into a calcified chamber, more or less filled with particles of sand, which are voluntarily placed in position by the animal soon after casting its exuvium.' But while the higher Podophthalma have the organ of hearing thus placed, there are some-the Myside-which, extraordinary to relate, carry it in the tail (see Plate XIII.). In some of the Amphipoda otoliths have been detected in connection with the brain, not in, but behind the antennæ. In general, the antennæ are furnished with delicate plumose hairs, the vibrations of which assist in the conveyance of sound to the auditory nerves. Similar hairs in the Mysidæ are connected with the caudal otoliths. The principal flagellum of the upper antennæ is frequently furnished with a number of smooth setæ or filaments, which were at one time described as auditory cilia, though there was nothing to support this guess at their function, and though the term cilium was inappropriate to the shape of these rod-like membranous filaments. It was noticed by various naturalists of eminence that the setæ of this form were much more abundant in the adult males than in the young males or in the females. Leydig supposed them to be not auditory but olfactory organs, and Fritz Müller independently came to the same conclusion, adducing in support of it their stronger development in the males, as in other cases male animals are guided by the scent in pursuit of the females. It can scarcely be said that their olfactory function is as yet absolutely proved, but they are evidently not well placed to serve the sense of taste, and for the senses of sight, hearing, and touch, there are other organs much better adapted, so that these glassy filaments, to be sensory organs at all, are in a manner forced back upon the sense of smell.

The secondary or inner flagellum, according to Dr. Boas, is wanting in all genuine Nauplii-that is, in the earliest larval stage of the crustacean. Its after development conforms to no known rule, since in some species it is not found at all, in others it is only rudimentary, whereas, on the other hand, among the Macrura it is not unfrequently much longer than the outer flagellum. Mr.

Spence Bate suggests that its function may consist in protecting and keeping clean the mass of cilia and filaments attached to the outer branch. In some genera of the Macrura, for example Palamon and Alpheus, the principal flagellum divides at some distance from its base into two branches. In the Squillidæ, also, there are three flagella.

3. The third segment carries the outer, under, or second antennæ, sometimes called simply the antennæ in distinction from 'antennules.' They are rarely absent, as in the females of some Amphipoda. More often they are strongly developed, in some instances exceeding in length all the rest of the animal which carries them, the joints of the flagellum or lash being then very numerous. In the Malacostraca the peduncular portion embraces the first five joints. The exopod, when developed, as it generally is in the Macrura, very commonly has the form of a thin plate known as the antennal scale, in Latin squama, while those who love long words are privileged to call it the scaphocerite. When laterally extended this broad scale must assist in keeping the animal upright in the water, a position which would otherwise with difficulty be maintained by long-bodied forms. In some Crustacea, the scale, though present, has not a laminar character, and it is then spoken of as the acicle. The first, or basal joint of the peduncle, is not unfrequently soldered to the wall of the head, and very often carries a tubercle in connection with the 'green gland,' of which the function is supposed to be renal, though it has not been with certainty determined. That this tubercle is of some importance may be inferred from the fact that in some cases where the antenna itself is obsolete the tubercle persists.

It is not a little singular that up to thirty years ago or later, many naturalists of eminence regarded this tubercle as connected with the auditory apparatus, which they assigned to the base not of the first but of the second antennæ. Milne-Edwards in 1834 refers to the researches of M. Savart, who had discovered that the stretching of a fine elastic membrane over an opening was one of the circumstances best adapted to promote the appreciation of

sound. Just such a membrane Milne-Edwards considers to exist at the base of the antennæ now under discussion, and in some of the Brachyura he and his colleague Audouin had investigated an inner apparatus capable of increasing the tension of the disk at the will of the animal, an arrangement which he compares with that of the auditory ossicles and the tympanic membrane of the human ear. It is only with reluctance that this description of a natural telephone can be relinquished. In some species, such as the common rock lobster, Palinurus vulgaris, there is a stridulating apparatus in the basal joints of the second antennæ, and it is obviously unlikely that a sound-producing organ should have been developed in an animal's ear.

4. The fourth or mandibular segment is of great importance, since from this, or from it in conjunction with the preceding segment, the carapace is developed. Its appendages also, the mandibles, yield in value to very few of the other organs. In form they vary extremely, but are for the most part of powerful structure. Their edges meet over the mouth-opening between the upper lip and the lower. The trunk of the mandible is frequently massive, with a projecting, finely denticulate, grinding surface called the molar tubercle, and a thick or thin dentate cutting edge, often having also a variety of spines between these two processes. It is not seldom surmounted by a narrow piece, commonly called its palp, which never in the Malacostraca consists of more than three joints. Very rarely, and only among the Entomostraca, one of the joints of the palp has an outgrowth supposed to represent the exopod. Since theoretically the exopod always arises from the second joint of an appendage, it is argued that the trunk of the mandible must represent the first joint. But to this it may be answered that the exceptional outgrowth just mentioned is perhaps not an exopod, and that at any rate in the first antennæ there is a similar outgrowth from the third joint. In Euchata glacialis, Hansen, and some other Entomostraca, the mandibular palp divides into two branches from its second joint. Seeing that the first joint of a crustacean appendage is very rarely of large