however, is the fact; six well developed ganglions distribute nerves to the muscular fibres of the lobster's tail; non-ganglionic columns supply the sensitive tail of the hermit-crab. One ganglion, indeed, is present in the Pagurus, but both its situation and office alike militate against the hypothesis of its special subserviency to sensation: it is developed upon the end of the smooth abdominal chords, and seems to have been called into existence solely to regulate the actions of the muscles of the claspers by which the hermit keeps firm hold of the columella of its borrowed dwelling.

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The general progress of the development of the nervous system in the Crustacea has been, as we have seen, attended with increased size, and diminished numbers of its central or ganglionic nerves. The divisions of each pair of ganglia first coalesce by transverse approximation distinct pairs of ganglia approximate longitudinally, conjoining as usual from behind forwards: confluent groups of ganglia are next found in definite parts of the body, as on the thorax of those species which have special developments and uses for particular legs. In the crab, in which the general form of the body attains its most compact form (fig. 90.), the ventral nervous trunks

are concentrated into one large oval ganglion(g), from which the nerves radiate to all parts of the trunk, the legs, and the short tail.

This condition of the nervous system has been described by Cuvier in the common crab, and is illustrated by Mr. Swan's dissections, from which his beautiful plates have been taken. The corre

sponding structure of the nervous system is also well displayed by Audouin and Edwards in the Maia. An analogous concentration of the nervous system, but with interesting modifications, has been described by Professor Van der Hoeven*, in the Limulus or King-crab, the most gigantic form of the Entomostracous tribe, and probably the

only existing genus from which we may derive an insight into the organisation of the extinct Trilobitic Crustaceans. This consideration has induced me to select for the exercise of the peculiar skill of Mr. Goadby, our anatomical assistant, the well preserved specimens of Limulus for which the College is indebted to Mr. Boott of Boston, U. S. In these beautiful dissections by Mr. Goadby, the details of the nervous system are clearly displayed, and will, with the rest of the Anatomy of the Limulus, be published by the Council of the College.

Three principal divisions of the nervous system of the Crustacea may be defined according to the views which I entertain of their functions. Thus, admitting, from analogy, that the supra-oesophageal ganglionic centre (figs. 89. and 90. c) is that in which true sensation and volition reside, then those nervous filaments which are exclusively connected therewith, and some of which would seem to extend the whole length of the animal along the dorsal aspect of the ganglionic columns, would form with their ganglionic centre the true sensori-volitional system; whilst any other ganglions superadded to the abdominal columns, with the nervous filaments terminating in or originating from them, would constitute the system for the automatic reception and reflection of stimuli. The stomato-gastric nerves, connected partly with the brain and partly with the oesophageal columns, will form a third system analogous to the great sympathetic or organic nerves of the Vertebrata. In these views I coincide with the ingenious physiologist, Dr. Carpenter, and shall feel happy if their accuracy and soundness have received any additional proof from the facts of Comparative Anatomy, which, in the Hunterian Lectures of 1842, were for the first time brought to bear upon this interesting problem.

The sense of touch can be but very feebly exercised by the common integument of the Crustacea, can hardly, indeed, exist except in those parts of the surface of the body which remains soft and undefended by the hard crust, such as the joints of the under part of the body, and the surface of the soft tail in the hermit crabs. The fine hairs which project from many parts of the integument may compensate for its low endowment of the tactile sense: the two pairs of jointed antennæ (fig. 90. a) are instruments fitted for the most delicate exploration; and the smaller but similar organs attached to the jaws, and called palpi, may also receive some impressions analogous to those of savour or smell. The Crustacea have no true tongue, but the sensations of the membrane lining the interior of the mouth and the oesophagus may guide them in the selection which they make of objects of food.

The sense of hearing is referred to a cavity with a round orifice

closed by a membrane, excavated in the first joint of the second pair of antennæ, in the lobster and other Macroura. In most of the Brachiura the membrane is stated by Dr. Edwards to be replaced by a small moveable calcareous disc, which is pierced with a small oval opening, over which there is stretched a thin and elastic membrane. The external opening of the ear is closed by this bony disc. A second small plate is so situated as to regulate the tension of the auditory membrane, whilst the rigid stem of the antennæ, in which the whole organ is situated, is well adapted to render the auditory vibrations more distinctly perceptible. These vibrations are conveyed through the medium of a vesicle filled with fluid to a branch of the antennal nerve which expands in the vesicle.

With respect to the organ of vision, we find in the class Crustacea a most extensive and interesting series of gradations, leading from the sessile median eye-speck to two distinct eyes, provided with all the essential optical apparatus and placed upon moveable peduncles. Ocelli ɔr stemmata are combined with compound eyes in the same species in certain Entomostracans, as Apus and Limulus. A transparent speck of the integument forms the cornea of the ocellus, immediately behind which there is a spherical crystalline body in contact with a gelatinous or vitreous humour, upon which the extremity of the optic nerve expands a layer of dark pigmentum covers all these parts with the exception of the cornea. In the compound eyes of Daphnia, the smooth undivided cornea protects and transmits the rays of light to an aggregation of small ocelli, each of which is lodged in a pigmental cell: the similarly constructed compound eye of the active little Branchipus is supported on a short moveable peduncle.

The large lateral compound eyes of the Limulus are sessile; the cornea is divided into a considerable number of small circular facets, each of which corresponds to an ocellus; and the optic nerve, after its long course as a simple chord, divides near the eye into a pencil of fine filaments, which severally receive the impressions from their respective ocelli, of the aggregate of which the large lateral eye is composed the two small simple median eyes, which are almost in contact, command the space before the head, which is out of the range of the large compound eyes. Each simple eye receives its distinct nerve from the anterior apex of the corresponding cerebral lobe.

In the sessile eyes of the Edriophthalma, as, for example, in the Serolis, the inner layer only of the cornea is divided into hexagonal facets, corresponding with the number of the conical crystalline lenses of the compound eye. But in the Trilobites, the cornea presents the same subdivided surface as in the Limulus; and the position of the two eyes agrees with that of the corresponding compound pair

in the large existing Entomostracan. The eyes are more elevated in the Trilobites. In the Asaphus caudatus the cornea is divided into at least 400 compartments, each supporting a circular prominence : its general form is that of the frustum of a cone incomplete towards the middle line of the head, but commanding so much of the horizon in other directions, that where the distinct vision of one eye ceases, that of the other begins.

In the mandibulate Crustaceans, distinguished by having their compound eyes supported on moveable peduncles, the form of the corneal facets varies; they are square in the river craw-fish, hexagonal in the hermit and common crabs. There is a conical crystalline lens behind each facet imbedded in a small vitreous humour, upon which the optic filament expands, and each ocellus is lodged in a pigmental cell, which likewise covers the bulb of the optic nerve; the cavity containing the compound eye is closed behind by a membrane continuous with the inner layer of epiderm, and pierced for the passage of the optic nerve (fig. 90. e). In the Podophthalmous Crustacea there is generally a spacious furrow or cavity, in which the eye and its peduncle can be lodged and protected, and it is termed the orbit. In one or two species the eye-stalks project beyond the margins of the carapace.

One of the most valuable and interesting results of the study of the comparative anatomy of the eye in the Crustacea is the insight which the fossilised remains of similarly constructed organs of vision in the extinct Crustacea have given respecting the state of the world at the time when they existed; and I cannot better conclude the present discourse than in the eloquent language of the geologist who first taught the value of the evidence in question. The eyes of the Trilobites of the transition rocks, and those of their nearest congeners, the fossil Limuli from the Carboniferous series, " give information," says Dr. Buckland, "regarding the condition of the ancient sea and ancient atmosphere, and the relations of both these media to light, at the remote period when the earliest marine animals were furnished with instruments of vision in which the minute optical adaptations were the same that impart the perception of light to Crustaceans now living at the bottom of the sea.

"With respect to the waters wherein the Trilobites maintained their existence throughout the entire period of the transition formation, we conclude that they could not have been that imaginary turbid and compound chaotic fluid, from the precipitates of which some geologists have supposed the materials of the surface of the earth to be derived; because the structure of the eyes of these animals is such, that any kind of fluid in which they could have been sufficient at the bottom, must have been pure and transparent enough to allow the passage of

light to organs of vision, the nature of which is so fully disclosed by the state of perfection in which they are preserved. With regard to the atmosphere, also, we infer that had it differed materially from its actual condition, it might have so far affected the rays of light, that a corresponding difference from the eyes of existing Crustaceans would have been found in the organs on which the impressions of such rays were then received.

Regarding light itself, also, we learn, from the resemblance of these most ancient organisations to existing eyes, that the mutual relations of light to the eye, and of the eye to light, were the same at the time when Crustaceans, endowed with the faculty of vision, were first placed at the bottom of the primeval seas, as at the present moment.

"Thus we find among the earliest organic remains, an optical instrument of most curious construction, adapted to produce vision of a peculiar kind, in the then existing representatives of one great class in the articulated division of the Animal Kingdom. We do not find this instrument passing onwards, as it were, through a series of experimental changes from more simple into more complex forms; it was created at the very first, in the fulness of perfect adaptation to the uses and condition of the class of creatures to which this kind of eye has ever been, and is still, appropriate."

LECTURE XV.

CRUSTACEA.

THE Limuli, which form the only genus of large Crustaceans represented by species which co-existed with the Trilobites, differ from all other living Crustacea in their organs of mastication, which are the modified basal joints of the five posterior pairs of legs: the first small pair serve to bring the objects of food to the mouth; they are supported on a rudimental labrum. In the Asaphus platycephalus Mr. Stokes has discovered a distinct subquadrate labrum deeply emarginate anteriorly; the nearest approach to this, the only known part of the trophi of the Trilobites, seems to be made by the entomostracous genus Apus, in which, however, the labrum is truncated. A few of the lowest organised Crustacea, as Caligus, Nymphon, and Pycnogonon, obtain their aliment, like the Epizoa, by suction.

In the Malacostracous Crustacea the mouth is closed by a small and