laria, it would only be necessary to shorten the œsophagus a little more, and then the wall of the corpus adiposum would be immediately attached to that of the body. So far, therefore, as concerns the corpus adiposum and the œsophagus, Sphærularia agrees neither with Gordius nor Mermis, nor indeed with one more than the other, since, if it agrees with Mermis albicans in the double series of large fat-cells, it has no œsophagus, and in this respect more nearly resembles Gordius.
Sphærularia agrees with Gordius in the possession of a terminal vulva, but differs both from that genus and from Mermis in having only a single ovary. As regards the development of the young, Sphærularia resembles Gordius in undergoing a metamorphosis; but with this remarkable difference, that whilst the former begins with the filiform or Nematoid condition, the latter ends with it. Mermis, on the other hand, undergoes no metamorphosis; in all stages of development this worm, like the embryo of Sphærularia and the adult of Gordius, is filiform and Nematoid; so that we may say of the three genera, that the metamorphosis is progressive in Gordius, absent in Mermis, and retrogressive in Sphærularia.
On the whole, it is, I think, evident that Sphærularia constitutes a group equivalent to Gordius or Mermis, and indeed farther removed from them than they are from one another. Omitting, then, those points as to which, from the imperfection of our knowledge, no conclusions are at present attainable, we shall get the following as the principal characteristics of the three genera:—
Mermis albicans.—Skin partially covered with papillæ. Œsophagus long, contained in an outer tube, within the outer membrane of which is a series of nuclei, at sub-equal distances. No intestine or anus. Organs of excretion three in number, and occupying the ventral and lateral lines of the body. Ovary double; vulva opening at the middle of the body. No metamorphosis, the young being filiform. Males of moderate size, free. Spiculæ two in number.
Gordius.—Skin smooth, or in part provided with short spines. Œsophagus very short. Corpus adiposum containing several series of cells. No intestine nor anus. Ovary double; vulva terminal. Metamorphosis progressive, the young not being filiform. Males of moderate size, free. No spiculæ.
Sphærularia.—Skin covered with spherules. Œsophagus wanting. Corpus adiposum consisting of a double series of large cells. No intestine nor anus. Ovary single ; vulva terminal. Metamorphosis retrogressive, the young being filiform. Males very minute, attached to the females. No spiculæ.
Thus, then, we see that Gordius and Mermis differ very materially from one another, while Sphærularia departs even more from the common type, and indeed agrees with the other two in little except the absence of an anus, and the very peculiar corpus adiposum. M. Schneider is therefore, no doubt, right in proposing to divide the Gordiaceæ into two families, to which we must now add a third—Sphærulariaceæ, for the genus Sphærularia. For the present we must leave the characters
