mollusc has shown that in early life there is a short compressed body, destitute of any lateral appendages, and presenting no repetition of segments.

In the skeleton of the vertebrata, it is necessary to observe the relations of its parts to the neural and hæmal organs, which are protected by it. The superior and inferior region of the molluscan animal must also be determined in order that the relations of parts in the archetype may be understood.

In vertebrata, the dorsal or superior aspect of the body corresponds to the position of the central mass of the nervous system, the hæmal being inferior. There is some difference of opinion as to the relations of the parts in the mollusca. Adopting the view that the neural side is also the lower or ventral, and the hæmal the

The archetype mollusc is supposed to be bilaterally symmetrical; the relations of the different parts will be understood from the accompanyng figure. The hæmal region (H) corresponds to that where the heart (h) is situated; the opposite is termed neural, and the great nervous centres (n n n) are usually placed in it. The anterior part of the body is marked by the position of the mouth, (m,) the posterior by the opposite opening of the alimentary canal or anus (a.) The lower or neural surface is usually called the foot, because generally employed as an organ of progression. The foot may be divided into three portions, the propodium (pp) or forefoot; the mesopodium (ms) or middle foot; and the metapodium (mt) or hind foot. The upper part of the foot, or middle region of the body, sometimes is prolonged into a fold or enlargement on each side below the point of junction of the hæmal and neural regions; this prolongation is called epipodium, (ep.) On the lateral and superior part of the head are two pairs of appendages, the eyes and tentacles. The part usually called mantle or pallium in mollusca, consists of a free fold of the skin either behind or in front of the anus. In the figure the branchiæ or gills (b) lie behind the heart, (h.) There are two principal modifications of this common plan depending mainly on the relative development of certain parts of the hæmal region. The portion of it in front of the anus is called abdomen, that behind it is called post-abdomen. Excessive deve

lopment of the former, accompa

FIG. 46. *

FIG. 46. Neural modification of archetype mollusc. The mouth, stomach, and alimentary canal are shaded. Here the part of the hæmal region above, or in front of the canal opening, is highly developed; the alimentary canal having a concavity toward the neural surface

nied by a bend of the intestine into it, (the concave part of which is directed downwards, or towards the neural surface,) constitutes a neural flexure. When the postabdomen becomes developed in the same way, the open part of the intestinal bend will be directed towards the hæmal surface, giving rise to a hæmal flexure.

FIG. 47. *

Professor Huxley considers, therefore, that there are two primary modifications of the molluscan archetype, which may be termed the Neural and Hæmal plans.

The presence or absence of a shell is of minor importance, and does not affect the relations of the archetype; all mollusca, therefore, may be referred to the same common typical form.

The cuttle-fish, with its formidable prehensile arms and beak; the singular Clio; the sluggish oyster; the more active pecten or clam; the destructive teredo or ship-worm; those expert tunnel-makers and borers, species of Pholas and others; the slug and garden snail; the pearl oyster ;-in a word, the almost endless forms of this great division of the Invertebrate sub-kingdom, may all be considered as framed after the same model, and we shall find that certain modifications of it have undoubtedly reference to the habits and mode of life of the animals.

FIG. 47. Hæmal modifications of archetype mollusc. Shews excessive development of post-abdomen, the part behind or below the anus. The alimentary canal has a flexure toward the hæmal region; in the fig. the heart is seen in the concavity of the flexure.

SECT. II.-MODIFICATIONS OF THE ARCHETYPE MOLLUSC.

Cephalopoda, or cuttle-fishes. These remarkable animals are usually placed in the foremost ranks of the molluscan type, and they present several interesting points of structure. The appendages, (whose position has given rise to the name Cephalopoda, or head-footed,) provided with a greater or less number of discs, each acting as a sucker, enabling them to retain their living prey and resist its struggles; their formidable beak-like jaws, by which they tear their prey in pieces; their bag, from which they explode an inky cloud, under cover of which they escape from their pursuers; their funnel, which serves as a discharge-pipe for water which has been in contact with the gills, and which, by the force of its escape, assists in aquatic progression; their highlydeveloped and curiously-constructed eyes-all give them a high degree of prominence in the estimation of the naturalist. They are pre-eminently the Felidæ of the ocean lying in wait for living prey; lurking in secrecy to spring on it; feeding chiefly in the twilight or at night; while their strength and rapidity of movement render them formidable enemies to many of their fellowinhabitants of the ocean. They are, moreover, the chameleons of the deep, having the power of rapidly changing the colour of their skin as emergencies require. What special modifications do they present, as departures from the model? and what relations do such bear to the habits of these animals? These are questions which may be now briefly examined, so far as the results arrived at by observers enable us to speak.

It is admitted that the development of all animals is subject to strict law, and the results of inquiries in this direction enable us to indicate the real nature of parts

whose homology, in reference to the archetype, may seem difficult to solve in the fully matured condition.

It has been already stated that the Cephalopods are so named from the position of certain organs, which, although chiefly employed for prehension and retention of prey, are nevertheless also capable of being used as means of progression on a hard surface. Designations of parts are not always in strict accordance with their true. nature, but it so happens in this particular instance, that the term Cephalopod is homologically correct, for the appendages which surround the fore-part of the animal

in reality correspond to the lower surface or foot, being actually lateral appendages of that part. These organs vary in number; in some species there are eight, in others ten. In the well-known Argonaut, two of the appendages are webbed, so as to present considerable extent of surface. These were mt described by Aristotle as the sails of the animal, which, in fine weather, and when floating on the surface, it expanded and raised to catch the wind--a description which, as it is now well known, does not indicate the true use of these parts; for their function is to form the shell, and progression is accomplished by the forcible ejection of water from the funnel, the animal being urged on its course by the recoil.

Pp

FIG. 48. *

*FIG. 48. Plan of cuttle-fish, to shew its relation to the archetype. pp, ms, ml, the parts of the foot modified to form the arms which surround the head; ep, epipodium forming the funnel through which water is discharged. The alimentary canal and heart will be seen in the middle of the shaded part of this figure.