l'hypothèse d'après laquelle les matériaux destinés à fournir à la moitié droite du corps siègeraient dans l'un des hémisphères latéraux de l'œuf, tandis que l'hémisphère ovulaire gauche engendrerait tous les organes de la moitié gauche du corps."

If our ideas of the promorphological relations of the ovum are well founded, it will be seen that we have a very satisfactory foundation for the opinion, first suggested by Balfour (Comp. Emb., II., p. 312), that the neural surface is identical throughout the metazoa. It is hardly necessary to add, that this view is in perfect accord with the theory of concrescence before mentioned. Indeed, it is difficult to see how one can hold to the former, and deny the latter.

EXPLANATION OF FIGURES.

Figs. 1-5, Ctenolabrus. Fig. 6, Ps. oblongus.

All magnified 280 diameters.

Fig. 1. Blastodisc seen from the inner surface. The Arabic numerals give the order of the cleavage-planes; the shading indicates that portion of the floor of the marginal cells which rests on the yolk. a, b, c, d, central cells; cl, boundary of the cleavage-cavity; zy, plane of the vertical section seen in Fig. 2.

Fig. 2. Transverse section of the 16-cell stage, in the plane indicated by the dotted line (xy) in Fig. 1. p, periblast; bc, cleavagecavity.

Fig. 3. Section of the blastodisc one hour after the 16-cell stage. The marginal cells are shaded.

Fig. 4. Two hours after the 16-cell stage. ep, epidermal layer. Fig. 5. Three hours after the 16-cell stage. The marginal cells assuming the form of periblastic cells.

Fig. 6. Transverse section at the time when the entodermic ring en, basis of the future entoderm.

appears.

V.

CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY OF THE MUSEUM OF COMPARATIVE ZOOLOGY

AT HARVARD COLLEGE.

No. IV. THE EXTERNAL MORPHOLOGY OF THE LEECH.

By C. O. WHITMAN.

Communicated June 11th, 1884, by Alexander Agassiz.

THERE is no invertebrate animal about which more has been written than about the Medicinal Leech; but, as Dalyell long ago remarked, "it does not appear that the history of the leech has advanced in proportion to the number of the literati who have rendered it the subject of discussion." As a considerable share of the work done in this direction is purely systematic, it is somewhat surprising that not a single description of any Hirudo has been given with sufficient accuracy and completeness for a close comparison of even its more important external characters with those of other species. More than this, it would be impossible, from the many monographs, memoirs, and stray papers devoted to this subject, to patch up a description that would fully meet the requirements for a critical comparison of any two species. By far the greater number of species-diagnoses that have been showered upon us from time to time have been so superficially and slovenly done, that it would probably puzzle the perpetrators to identify the species they profess to have described. Some of the more important diagnostic characters have been either entirely ignored, or given with such vagueness that they are of little service in identification, and absolutely worthless for comparative purposes. No uniform mode of counting the rings has been adopted; and, judg ing from the descriptions themselves, systematic writers have, for the most part, failed to place much value on the rings in the determination of species and genera. No one appears to have suspected the existence of segmental sense-organs in the leech; much less the serial homology of the eyes with such organs. It is the object of this paper to make clear both these facts; and, at the same time, to show that the rings and somites form the only proper basis of classification. The latter fact will be brought out by a comparison of a few wellmarked genera.

It is a well-known fact that the end somites are incomplete in respect to the number of rings composing them; but the mode and extent of abbreviation have not hitherto been made the subject of careful study; and hence some of the more important generic characters have been entirely overlooked, and with them some points of the highest morphological interest. Hirudo is the best known and most widely distributed genus, and on this account forms a convenient standard of comparison. As the arrangement of the segmental senseorgans enables us to determine the exact number of somites composing the body, we may deal with this part of our subject first.

The Segmental Sense-organs of the Leech. The only sense-organs hitherto known in the Medicinal Leech are the five pairs of eyes and the so-called "goblet-shaped" organs located on the lip (cephalic lobe). A number of writers have noticed and described some small spots, occurring on every fifth ring of the body; and one author has suggested that they may have a respiratory function. These spots when examined closely with a low magnifying power will be found to be slight elevations with rounded summits; and for this reason, and because they are regularly disposed on the first ring of each somite, they may be called segmental papilla, a name which does not prejudge the question of their function.

In our large Pond Leech (Macrobdella Verrill), these papillæ are comparatively small; and the same may be said of the medicinal leeches of Europe and Japan, and their nearest allies, Hæmopis and Aulostoma. In some of the Asiatic medicinal leeches, for example, those of Saigon, Singapore (H. maculosa), Java (H. javanica), and Ceylon (H. multistriata), they are much larger, and have an oval form, with a median ridge or crest. In the land leeches they are very conspicuous, having the form of small cones with rounded summits.

In all the ten-eyed leeches of Japan, including both the land and fresh-water forms, twelve of these papillæ are found on the first ring of each complete somite, six on the dorsal and six on the ventral side. In most of the medicinal leeches, however, as well as in Hæmopis, Aulostoma, Macrobdella, etc., there are eight on the dorsal side, and six on the ventral.

A careful study of the arrangement of these papillæ in a large number of species, and of their histological structure, has brought out in a most conclusive manner their serial homology with the eyes; and has led, indirectly, to the recognition of some important points in regard to the metameric composition of the body of the leech. The arrangement of the papilla on the dorsal side is shown in Fig. 1.