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Show 1892.] SPECIES OF EARTHWORMS. 695 the large egg-sacs. Even Rosa, who is so anxious to minimize the affinities of the Moniligastridae to the lower Oligochaeta, admits that the size of the eggs-sacs is unknown in the Terricolae, though it is, he remarked, going rather too far to exclude Moniligaster from the Terricolae on these grounds. In Moniligaster beddardi no egg-sacs were found by Rosa ; but I do not think that this failure to find the structure in question is tantamount to a proof that they are non-existent in that species, as Rosa seems to imply. Anyhow they are large in the present species, and occupy at least three segments. Bourne, the first to discover these bodies, stated that they occupy in Moniligaster minutus segments xii.-xv. As to the ova of Moniligaster, Bourne says nothing about them save that there are ova in these sacs; the eggs in Moniligaster beddardi are, according to Rosa, very minute. In the species here under consideration the ova present a very remarkable character, unique among Earthworms: they are not particularly large, though, perhaps, larger than in many Earthworms ; the remarkable fact about them is that they are crowded with yolk-particles: to so great an extent is the yolk developed that the nucleus is by no means always apparent; the yolk-particles are moreover, as is shown in the accompanying figure (Plate XLV. fig. 1), of considerable size, quite as large as they are in eggs of a much greater size. This fact about the ova of Moniligaster bahamensis is of considerable interest. I pointed out some time since1 that the only distinguishing characters between the Megadrili and the Microdrili of Benham2, not alluded to by Benham himself, are the three following:- ' (1) Large size of ova. - (2) Clitellum consisting of only a single layer of cells. (3) Sexual maturity at a fixed period. In the three points mentioned the Microdrili (= Limicolae of Claparede minus Naids and Molosoma) differ from all the Megadrili or Earthworms. Now I have just pointed out that the ova of Moniligaster, although not so large as they are in tbe Microdrili, agree with them in having a great quantity of yolk, a character not found in any other Earthworm ; this is, at any rate, an indication of a step in the direction of the Microdrili, even if it be not held to be a point of close affinity with them. On a previous page I have pointed out that the structure of the clitellum is quite like that of the lower Oligochaeta in being made up of a single layer of epithelium only ; it may be added that in all possibility the sexual maturity is at a fixed period; this would account for the failure of so many investigators to find the clitellum ; Prof. Bourne tells me that he expects that the clitellum will be found at the proper season in all Moniligasters. It is difficult therefore to see on what grounds Moniligaster is to be referred to the Earthworms as opposed to various groups of the aquatic Oligochaeta. Rosa justly points out that Tetragonurus 1 " On the Anatomy of Ocnerodrilus (Eisen)," Trans. Boy. Soc. Edinb. xxx vi^ 2 " An Attempt to Classii'y Earthworms," Q. J. M. S. vol. xxxi. |
