helminthologists, class it with the true Nematodes. The absence of an organ is, however, not generally so important a character as its structure: thus, for instance, we see in insects, that the absence of wings is less significant than are the differences in their structure, so that we have wingless representatives of all the large orders. The characters which induce us to separate Sphæralaria, Gordius, and Mermis, from the Nematodes being principally negative, are not to my mind quite satisfactory. Schneider differs also so much from Meissner as to the anatomy of Gordius and Mermis, that it will be necessary to say a few words on this subject, before considering the affinities of Sphærularia. The so-called supra-œsophageal ganglion of Mermis he denies to be a portion of the nervous system at all, and considers it rather to be an œsophageal bulb, homologous with what is found in many Nematodes. If this be granted, the principal argument in favour of the nervous nature of the so-called peripheral nerve-system falls to the ground, and with it one of the principal differences between Sphærularia and Mermis. While, however, we know nothing about the nervous system of Sphærularia, and are in such a state of uncertainty as to that of Gordius and Mermis, it is evident that we cannot avail ourselves of it for the purposes of classification. Meissner's extraordinary account of the digestive organs in Mermis is well known. According to him, the œsophagus is open along one side, thus constituting a trough rather than a tube, which sends out from time to time lateral branches, each of which terminates in a spherical cavity, which he calls a stomach-cell. According to Schneider, however, the œsophagus is a closed tube, and the "stomach-cell" is only a round, firm body, containing a nucleated structure, but without any central cavity, or any communication with the fat-body. This "fat-body" is probably homologous with the intestine of ordinary Nematodes, but no cavity has been developed in it; and while Meissner describes thirty connecting tubes between it and the œsophagus, Schneider denies that any such junction takes place; the two organs lie side by side, but have no communication with one another.
Schneider exemplifies this by the case of Ascaris rigida, R., in which the œsophagus opens, not at the anterior end, but at the side of the intestine. If, he says, this condition were exaggerated, and the lateral connexion removed, we should have exactly the case of Mermis. In Mermis albicans the fat-body consists of two rows of large cells, as in Sphærularia; but in Gordius the cells are much smaller and more numerous, still, however, solidly filling the tube; while in Mermis nigrescens the cells are smaller, and only clothe the outer tube, and leave a large central cavity; thus completing the series, and giving us a most interesting gradation, connecting the corpus adiposum of Sphærularia with the ordinary intestine of any common Nematode. This corpus adiposum, therefore, is homologous, not with the whole intestinal canal of Nematodes, but only with the intestine; and we find, in fact, that in Gordius the œsophagus is very short, and opens at once into the anterior end of the corpus adiposum; so that, to pass from this genus to Sphæru-
