OCR Text |
Show 1900.] EARTHWORMS OF THE GENUS AMYNTAS. 6ll presence of additional sperm-sacs in segment xiii., and even in x., seems to be distinctive of a few species; but here it is not certain that variations do not occur. There are next to be considered a number of characters which will probably be of use when they are better known. At present they have to be used with great caution, and cannot be much respected unless they are based upon the examination of a large number of individuals. (1) The Genital Papillae.-It does seem that there are species (e. g. A. padasensis) in which these structures are entirely absent. But they appear likewise to be occasionally absent from specimens of species which normally possess them. In a few forms they are very constant in number and arrangement. In A. posthumus, for example, there are two pairs always (except in obvious asymmetrical monsters) in the same place. But elsewhere they vary enormously. To give one instance: I have examined many specimens of the common A. heterochcetus, and only once have I found a pair of small papillae in front of and behind the spermathecal pores, a common enough position for such papillae in other species. (2) The relations of the Spermatheca to its Muscular Duct.- Sometimes, as in A. forbesi and A. impudens, it is easy enough to distinguish the relatively large thick duct from a duct which is practically absent, the pouch being almost sessile. But other cases come so near to each other, and the amount of contraction may be so different, that the character is not always of practical value. The same is the case with the diverticulum. The long spirally-wound accessory pouch of A. musicus cannot be confounded with the rudimentary knob of A. iris; but the middle examples of the series cannot be so easily defined, and in most cases cannot be distinguished at all. The occasional existence of accessory diverticula upon the main diverticulum does not appear to be constant. Barely (A. ijimce) there is no diverticulum ; but it would be more satisfactory to be assured on the results of section-cutting that the absence is real and not merely apparent. (3) The length, thickness, and degree of curvature of the muscular duct of the glandular appendage of the sperm-duct is apparently a character of value; but, again, it has to be used with caution, chiefly because the descriptions extant are not always clear upon the point. Also the shape of the gland itself. This, however, does appear to vary considerably in degree of lobation and in size in the same species. (4) A fourth character which must be used with caution is the extent of the clitellum. Not indeed those cases where it extends beyond the normal segments, but its actual line of commencement and ending upon the xivth and xvith segments. It certainly varies a little in the same species. (5) It is quite possible, when more is known about the facts, that the point at which the sperm-duct becomes confluent with the duct of its terminal gland will prove to be a character of importance. (6) A n ornamentation of the setae has been observed in A. hetero- |