516 SUMMARY OF OUERENT RESEARCHES RELATING TO 



Anatomy of Hirudinea.* — ^Mr. C. O. Whitman has a preliminary 

 notice of some new facts about the Hirudinea. As a group, they are 

 characterized by the possession of segmental organs on the first ring of 

 every somite. The diffuse or non-metameric arrangement which is seen 

 in Nephelis and some other forms seems to have been acquired 

 secondarily. The author has shown that in all ten-eyed leeches the 

 eyes represent enlarged, more or less modified, segmental sense-organs ; 

 if this be true of other leeches, it would appear that the metameric 

 sense-organs are earlier in origin than the non-metameric. In two 

 species of Clepsine it has been seen that the segmental sense-organs 

 appear very early in the embryo, before the time of hatching, while the 

 scattered organs arise later. The labial sense-organs are serially homo- 

 logous with ventral sense-organs, as the author will soon show. Mr. 

 "Whitman's experience leads him to think that " most of our reputed 

 blind leeches will yet be made to bear testimony to the blindness of their 

 observers." In a new Japanese marine leech, Branchelliopsis, eyes appear 

 to be altogether wanting, but very careful search revealed the presence 

 of at least two pairs of eyes. They have so little pigment that they 

 cannot be seen from the surface, but the visual cells are there. Another 

 new genus, Piscicolaria, from the smaller lakes of Wisconsin, comes 

 nearer to being blind than any leech yet examined ; the only evidence of 

 an eye is a single large visual cell on either side of the head without a 

 trace of pigment-investment. The test of a leech eye is the presence of 

 visual cells ; these are the large clear cells of Leydig ; they always make 

 up the bulk of the eye, and in the Hirudo pattern they are the only cells 

 which are supplied by the optic nerve ; their main axis is generally, 

 though not invariably, parallel with the axis of the eye ; in Clepsine and 

 BrancTielliopsis the nucleus lies on the side exposed to the light, the 

 clear rod-like part of the cell being directed towards the pigment ; the 

 cells are practically inverted, the nerve-fibres entering at the nucleated 

 pole. The chief distinction between the different patterns of eye and 

 the typical sense-organ lies in the relative abundance of the clear 

 cells. 



The segmental sense-organs are double, both in structure and func- 

 tion ; there is an axial cluster of elongated cells, terminating at the 

 surface in minute hairs, and probably representing a tactile organ. 

 Around and beneath the tactile cells are the large clear visual cells, so 

 characteristic of the eye. We have, therefore, a visual and a tactile 

 organ combined, both derived from a common mass of indifferent 

 epidermal cells, and both supplied by fibres from a common nerve- 

 branch. Incredible as the double nature of these organs may at first 

 appear, there is no escape when we once understand the structure of the 

 eye in Clepsine. Both the eyes and the segmental sense-organs develope 

 as local thickenings of the epidermis, and at first the cells are alike in 

 form, size, and structure ; about the time the pigment begins to appear 

 the two sorts of sense-cells begin to show a difference in size, and an 

 indistinct boundary line appears between them. 



It is urged that the metameric arrangement of the sense-organs of 

 the Hirudinea is a matter of more importance than the latest writer on 

 the subject — Apathy — appears to imagine. The key to the analytical 

 study of the external form is to be found in the metameric disposition 



* Journal of Morpliology, ii. (1889) pp. 586-99. 



