modifications of the instruments are of commensurate import. The leaf-flies, the gall-insects, the saw-flies, and ichneumons, all present instruments varying in length and strength, according to the substance which each is intended to penetrate. Those of the leaf and gall-insects are just sufficient to allow them to penetrate vegetable tissues; neither is there any great force requisite to enable the ichneumon female to deposit her ova in the bodies of other insects, or in the cocoons of spiders, or in the eggs of butterflies. The saw-flies, which penetrate hard wood for a similar purpose, are provided with an apparatus of great power and admirable construction. In every special case there is some remarkable harmonious adaptation, so that by inspection of the apparatus we can ascertain the way in which the eggs are deposited. The elements concerned in boring are placed in pairs, and furnished with teeth on the edge and sides, the former serving as a saw, the latter as a rasp. This delicate instrument requires support when acting, as well as protection; and accordingly these desiderata are provided, and the sword does not more accurately fit the scabbard than are the respective parts of the ovipositor suited to each other, and to the habits of the insects.

Certain elements and appendages of the terminal part of the abdomen are transformed into a saw or file, or both, as the case may be, and others are fitted to give them strength and protection.1 It matters not what the size of the insect, whether the comparatively large Sirex or the very minute Ichneumon ovulorum,2 the general plan is the same, but in every instance presents some peculiarity adapted to the nidus selected by the species.

1 Lacaze-Duthiers, on Genital Armature of Insects, Annales des Sciences Naturelles, 1849, 1852.

2 This tiny insect deposits several ova in a single egg of a butterfly, the contents of which afford sufficient food, as well as protection, to all the young which are produced.

The formidable sting of the bee and of the wasp are examples of other modifications presenting no less beautiful harmony between organ and function. Generally speaking, the appendages of the abdominal segments are absent, or if present, very rudimentary, because not required in the economy of the insect. What we have

stated respecting the ovipositor and sting, affords proof that when certain appendages are necessary they are provided.

We find them, however, in other cases, furnished for a different purpose. The insects called skip-tails present remarkable examples of this. In the genus Lepisma, there is a pair of appendages attached to each abdominal segment. In Podura, and others, the singular tail-like organ consists of an elastic stem ending in two branches, like a fork and its handle. During repose this instrument is bent beneath the insect, and is lodged in a groove; when suddenly straightened the animal is thus enabled to spring a considerable distance. The handle of this forklike organ is believed to represent the sternal or lower part of an abdominal segment, the two prongs are stated to be the homologues of the lateral appendages.

We may finally, and very briefly, allude to a remarkable transformation of abdominal appendages in another class of the jointed invertebrata.

The web of the spider is constructed of delicate threads, which are given out by parts called spinnerets; these are organs consisting of two or more joints. The end of the spinneret is pierced with a great number of small holes, each of which gives out a drop of fluid which hardens in the air. The minute threads of each organ are joined to form one, and those of all the spinnerets again unite to form the apparently simple thread of the spider, which is therefore in reality complex. There appears

to be no doubt that the organs which produce the spider's thread are really abdominal appendages, thus singularly modified for the animal's convenience. They are composed of several joints, as limbs are, and in some species one pair of them-not being perforated nor furnished with an organ to produce the thread, and therefore apparently not needed-are nevertheless of interest to the zoologist, as indicating the real nature of the true spin

nerets.

CHAPTER VIII.

RADIATA.

SECT. I.-TYPICAL FORMS OF RADIATA.

THE Radiate type of animal structure, as the name indicates, is characterized by a tendency to repetition of parts round a centre. This division of the animal kingdom comprehends, on the one hand, the minute and soft hydra of our fresh waters, and, on the other, the hard and formidably-armed urchins of our seas.

At one time, many of the radiates were supposed to belong to the vegetable kingdom; more accurate observation has resolved the doubts respecting their nature, and demonstrated that they belong to the animal kingdom. It may be added, however, that still more recent discoveries have shewn that, in the mode of reproduction by buds and ova, they present a remarkable parallelism to plants. And here we see evidence that certain animals and plants have so much of unity of plan, as to shew that they have been constructed by the same Architect.

Our aim is to shew that, while there is adherence to a Radiate plan, there are departures from it on the one side and on the other-deviations which have reference to some end in the economy of the animal. We meet with difficulties in this as in other departments, but we doubt

not that as science advances, and our knowledge of their development, of their structure, and of their habits, becomes increased, additional proofs will accumulate in favour of our argument.

Professor Huxley has done good service in shewing the relations of certain Radiata, viz., Medusa, Physophoridæ, and Diphyde, belonging to the Acalepha, or sea-jellies, and Hydra and Sertulariadæ, placed among Hydroid Polyps. He considers them "members of one great group, organized upon one simple and uniform plan, and, even in their most complex and aberrant forms, reducible to the same type."1 Among Echinodermata,2 there is evident adhesion to a common type, while there is, at the same time, wide range in their general aspect. In some of the sea-urchins the body is almost spherical, in the sea-stars it is angular; but these extremes pass into each other by almost insensible gradations. Among the sea-urchins, Echinocyamus and others present a pentangular outline; in Asteriscus, one of the sea-stars, the general form is similar, the angles, however, being very indistinct. In Solaster, the angles are more prominent; in Asteracanthion, Ophidiaster, and Luidia, the angles are changed into true rays, and become more and more distinct from the body. In Ophiura, this separation into arms and body is complete, and in Euryale, the arms become very much branched. The flattening of the body also differs ;-in Palmipes membranaceus, we have a good example of extreme depression, while in some species of Oreaster, the arms are very much dilated, so as to present in section the form of an equilateral triangle. Among the sea-urchins we observe similar dif

1 Philosophical Transactions, 1849.

2 Some hold that the Echinodermata possess annulose or articulate characters. We here follow the views usually adopted respecting them.