In those with ten appendages, two are longer than the others, and serve as anchors to moor the body, or are darted out to capture prey beyond reach of the shorter

arms.

Allusion has been made to the functions assigned to the funnel; this part, so necessary in the economy of the animal, may be also referred to its corresponding part in the archetype. It is derived from the epipodium, upper foot, (Fig. 48. ep,) the posterior part only is considered by Professor Huxley as contributing to the formation of this important organ. "The mouth is thrust

back between the halves of the mesopodium, the propodium and mesopodium forming a continuous sheath— bearing tentacles-around the oral aperture. The two halves of the epipodium united form the funnel."*

Pteropoda.-The animals so denominated are generally of small size, but this is compensated for by their numbers. In the tropics, as well as in the Arctic seas, they abound, and, with other marine invertebrata, serve to stock the pasture-grounds of the great whales. The peculiar appendages, or lateral flaps, from which they derive their name, (Pteropoda, wing-footed,) are the principal means of progression by which they flit hither and thither whence they have been appropriately called the moths and butterflies of the ocean. As littoral productions they are not generally known, excepting from the shells of some which are occasionally cast up; but in the open sea, far from land, they are sufficiently familiar to the observant navigator.

In these interesting molluscs, the parts called fore, middle, and hind foot, are generally in a rudimentary condition, and the epipodium or upper foot forms the wing-like appendages so necessary in the act of progres

* Knight's English Cyclopædia, Art. Mollusca.

sion, and giving such a marked character to these animals. Cleodora, Euribia, Clio, Pneumodermon, and others, present each peculiar but minor modifications of the epipodium, doubtless in harmony with the habits of the respective species, but, nevertheless, essentially of the same nature, and performing the same general function.

The epipodium, which is but a narrow band in the archetype, appears, therefore, to attain its maximum of development in certain Pteropods, and forming wing-like appendages copiously traversed by strong muscular fibres, is admirably fitted to be employed as oars, and the testimony of observers confirms such idea respecting its use. -There are other mollusca not far removed in appearance from those just described, which also deserve to be noticed here as examples illustrative of the argument. They have been called Heteropoda. Like the Pteropoda, they are constituted for free progression in the water. The relations of their parts have been very fully examined by Professor Huxley in the Essay already quoted. The body in one genus, namely, Firola, is clear as crystal, so that all its internal organs can be distinctly seen, and the author quoted describes it "as hardly distinguishable in the water, except by the incessant flapping of its flattened. ventral appendage." The shape of this organ, by which the animal makes progression in the water, is that of a cheese-cutter; it is a modification of the propodium or fore-foot of the archetype, the other parts remaining rudimentary. In another genus, viz., Atlanta, progression in the water is accomplished by means of an appendage similar to that of Firola, and a modification of the same part, thus remarkably constituted to serve an important end in the economy of the animal. But Atlanta has the power of attaching itself to marine plants by means of a sucking disc placed behind the propodium;

this part is the mesopodium, which thus presents a modification different from that of the propodium, the one as well as the other, however, being admirably suited to its function. Moreover, the metapodium, or tail, as it is sometimes called, bears on its surface the hard body called operculum, which serves as a lid to close the mouth of the shell when the animal retreats into that appendage.

In Aplysia, or sea-hare, the epipodium is highly developed for a special purpose, namely, to assist in locomotion. Professor Huxley describes a tropical Aplysia as flying through the water in precisely the same way as a Pteropod would do. In Natica, we observe the mesopodium modified, to serve as a disc for locomotion by creeping; the metapodium bearing the operculum or lid which closes the mouth of the shell when the animal takes refuge in it.

Among Bivalves, as they are called, from the form of the protecting shell, we find numerous modifications of the neural surface in evident relation to the wants of the animal. In the oyster, destined to sedentary life, it is small; in Solen or razor-fish it is large, constituting the foot, which the animal employs as an effective means of burying itself in the loose sand. According to the views of some, the same part is actually so modified in its form, and in the nature of its constituent tissues, that it may be used as an instrument for perforating wood and rock. Whatever be the form or function of this necessary organ of the bivalve mollusc, it is supposed to correspond to the metapodium of the archetype.

Certain Gasteropodous mollusca are, when young, protected by a shell resembling that of the nautilus in miniature. At this stage they do not possess the power of creeping, but swim freely in the water-a provision

which secures their wide distribution, and gives rise to fresh colonies at a distance from the parent. At this early period of life they are provided with two wing-like appendages fringed with cilia; these are employed as oars, by which they move from place to place. The appendages in question are believed to correspond to the anterior part of the epipodium. This peculiar modification is, however, only a temporary arrangement; a time arrives when it is no longer needed; it then disappears, and the adult animal accomplishes progression on hard surfaces by means of the foot proper. The ciliated epipodium is provided for a temporary purpose, and when that is accomplished it disappears, to be superseded by another part.

It is therefore admitted, that all mollusca present traces of a common plan; and although in every instance it may not be possible to indicate with clearness and precision the special ends of the many modifications of the archetype, still, arguing from what we do know, it is not unreasonable to conclude that we have here independent members in harmony with each other, and conspiring to promote the wellbeing of the animal in its destined sphere of life.

CHAPTER VII.

ARTICULATA.

SECT. I.-HOMOTYPAL RINGS AND APPENDAGES.

WE now pass to the Articulate type of the Invertebrata, comprehending crabs, barnacles, insects, spiders, and others. These agree in one obvious charactertheir body consists of a series of similar or homotypal rings, which present almost endless variety in size, form, and other particulars, according to the habits of the species. The rings are generally, in the higher kinds at least, of more or less hard texture, giving support to appendages, and serving as points of attachment to numerous muscles, as well as protecting various important organs concerned in the function of sensation, motion, circulation, &c. They present us with examples of a highly-developed outside covering, technically called exoskeleton, the character of which varies as there is necessity in different parts, for variety of motion, for solidity, or for simple protection.

The endless diversity in form, and the exquisite beauty of colour and sculpture, exhibited by certain of the Articulata, have rendered them favourite objects of study, and their history has been in general very thoroughly

It is not our intention to discuss here all the classes of the Articulate type; a selection will suffice for our purpose.