of the blastoderm. As yet a complete series of investigations are wanting, but I have traced the steps of development sufficiently to allow me to state that the great procephalic lobes which exist in the half-developed embryo become folded inwards, and lie one on either side of the alimentary canal during the whole period of larval life, and that the nervous system of the larva, by the modification of which that of the imago is developed, is formed as a thickening of that portion of them which originally bounds the primitive fork at the anterior extremity of the ventral groove. These involuted procephalic lobes-and they are nothing else-form a portion of the imaginal disks of Weissmann, whilst the eyes, antennæ, and mouthorgans, are ultimately developed from cellular outgrowths at the bases of the same structures, just as they are in the Crustacea.

The nerve centres of the higher insects, such as the blow-fly and the higher social Hymenoptera, are even more remarkably like those of the Vertebrata, for a pair of cerebroid ganglia are developed in them in the most anterior portion of the procephalic lobes. These cerebroid ganglia were first pointed out in the Hymenoptera by Dujardin. The ganglia in question are pedunculated convoluted nerve centres, situated at the anterior and upper part of the prestomal nerve centres; they are united by a well-marked commissure, never give off nerves directly, and are further connected with the great ganglia of sensation by peduncles, just as the cerebrum is in the Vertebrata.

At present I have not, it is true, succeeded in tracing the development of these cerebroid ganglia, but do not doubt I shall ultimately do so. From their position it is not difficult to see that they must originate from the anterior (in after-development posterior by flexion) portion of the procephalic lobes.

Having thus pointed out to you very briefly the present condition of research as to the relation of the nervous system of the Arthropoda and Vertebrata, I will draw your attention to the relation of the nervous system of the higher Annulosa to that of the Annelida and Annuloida generally.

The nervous system of the Echinodermata will serve well as a starting point. It consists, as you well know, of a band of nerve tissue surrounding the pharynx, and composed of nerve fibres with nerve cells scattered amongst them. From this circular ganglion band radiating ganglion bands (usually five in number), having the same structure, pass with the ambulacral vessels towards the antambulacral, or, as it is sometimes called, dorsal region, that is, towards the anal pole of the animal.

If I may be allowed to speculate on this subject, it seems to me probable that the primitive nerve ring is connected with that involution of the dorsal integument of the embryo from which the ambulacral system is developed, and is hence derived from an external

* Ann. Sc. Nat.,' Series iii., T. xviii., p. 231.

layer of the embryo. I throw this out, however, as a mere suggestion.

The next type of nervous system to which I would draw your attention is common amongst the Scolecida, especially those which, like the Nematoids and Gordiaceæ, bear many resemblances to the Echinodermata. The nervous system consists of a ring of nerve tissue around the pharynx, histologically like that of an Echinoderm, and closely related to the cellular layer of the integument, of which I believe it to be a differentiation. From this two or more bands of similar structure pass, according to my own observations,* to the anal pole of the body. This seems to be the case in the Ascaris at least, where I have traced short rudimentary intermediate bands passing back from the oral ring between the main nerve bands; these apparently represent the three missing radiating nerves of the typical Echinoderm. Some forms, as Mermis, have three instead of two well-developed nerves passing back from the oesophageal ring.

In the earth-worm, according to the elaborate researches of E. Claparède (whose much-lamented death is one of the greatest losses zoology has met for years), the structure of the nervous system follows a precisely similar type. In this case, however, only two bands pass back from the oesophageal nerve ring, and these are closely united and lie in the middle line on the ventral aspect of the body. Histologically, their structure is nearly like that of the Echinodermata, but the nerve cells show a tendency to become aggregated in places; this is the first indication of the formation of distinct ganglia.

The transition from this condition to that observed in the higher Annelida, Insecta, and Crustacea, is not difficult. The nerve cells become collected in separate groups, or ganglia, and the remaining portions of the cords form the longitudinal commissures between them. The nervous system also becomes more central by the folding inward of the serous layer of the blastoderm to form the internal skin growths already mentioned. Thus we arrive at the highest forms, and we trace connecting links between the whole great series of organisms grouped under the sub-kingdoms Annuloida and Annulosa.

Before concluding, I cannot help referring to the visceral system of nerves so well marked in many Articulates, which is clearly developed from the mesoblast, as the sympathetic system is in the Vertebrata. I would draw attention to the great similarity of the complex visceral plexuses and the chain of visceral ganglia in the two great sub-kingdoms. These latter are undoubtedly differently placed, but their development from the mesoblast is a strong indication that they are to some extent homologous as well as analogous systems.

* Monthly Microscopical Journal,' January, 1871.

Acarellus muscs.

Development of Nervous

Systemin Arthropoda.

3

W.West & Co.lith.

V.-Notes on New Acarelli. By J. G. TATEM.

PLATE XL. (Upper portion).

In exhibiting balsam-mounted slides of new species of Acari, I wish to call the attention of the members to a fact of great significance in the life history of the creatures, and one, too, of much value generically considered. We all know that the young of the Acarine division of the Arachnid family are excluded from the ovum in an immature condition, possessing but three pairs of legs, though they acquire the normal four pairs after one or more subsequent moults. The common harvest bug, of which the adult form has yet to be recognized, is but a too familiar example of this hexapod stage of Acarine existence. The subjects I have embalmed have, however, entered upon the mite world in a still more imperfect state-they have but two pairs of legs!-but to prove that they would ultimately be as capable as any of their congeners of exercising four pairs, they have two posterior pairs neatly packed up in their trunks ready for evolution in the progress of growth!

Two years since, at the July meeting of the Entomological Society, Professor Westwood gave descriptions of two species of minute four-legged Acari, the one detected in the unopened buds of black currant trees, the other in pustules on the leaves of pear trees, referring also to a third which had been previously found in France. He considers them a distinct tribe, and that they must at all events be placed in a separate genus, for which he proposed the name of Acarellus. To this group our specimens unquestionably belong, and although their obviously imperfect development more than justifies the suspicion that the genus is based on immature forms alone, it still constitutes a good and natural division of the family.

In that example to which I have ventured to assign the specific name of A. pulicis, Plate XL. (its habitat having been the abdominal cavity of a dead flea), the hinder pairs of limbs are almost perfectly and completely formed, but still included within the cephalo-thorax, evidently awaiting the nearly approaching moult for extrication; while in the other, which on like grounds I have called A. muscæ, Plate XL. (detected clinging tenaciously in company with half-adozen others to the thorax of a small dipteron), the hinder pairs are more rudimentary, and with its much smaller size indicates an earlier stage of existence and more remote ecdysis. In both instances, however, it is quite apparent that in the process of development they would sooner or later have transferred themselves from the new genus Acarellus to that of Acarus proper.

Of their life histories I have no knowledge. Discovered by accident, and not until after many hours of maceration in liq. potassæ,