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Show 12 DR.T. S. COBBOLD ON A NEW ENTOZOON. [Jan. 13, the genus as Tricheilonema; and, indeed, 1 had already so written it, when I afterward found that the late C. M . Diesing had already employed the same generic title for a parasite of a somewhat different type. In his final revision of the Nematoda, communicated to the Vienna Academy in 1860, he places this Nematode (described in his ' Systema Helminthum' as a species of Filaria) as the type of his new genus Tricheilonema; whilst, unfortunately, in the Introduction or Conspectus of the same revision, this genus, Tricheilonema, appears under the synonym of Schizocheilonema. This complication of terms is vexatious-the more so since his term Tricheilonema would have been much more suitable for the designation of our new parasite than for the particular form of Filaria there described as having been obtained from the oesophagus of an Austrian Snake. On the other hand, since Diesing's ready method makes no pretentions towards a natural classification of the Entozoa, and since, also, in the present state of our knowledge, it is much more convenient to utilize his system of arrangement than those of other systematists, there is the less reason to regret the necessary introduction of a new generic term. If Schneider's system be more natural, it is, at all events, much less complete. Without further apology, therefore, on this score, I may also remark upon the great difficulties surrounding a natural classification of the parasitic Nematodes. The variety of characters they display, especially at different stages of their growth, the remarkable disparity of size occasionally shown by the sexes, to say nothing of the still more astonishing fact that the adult female Entozoon may itself occur in two totally distinct forms-all these peculiarities, not to mention many others (associated with or depending upon their migratory habits), add to the difficulties of taxonomy. These instances of dimorphism, it is true, are now no longer believed to be confined to the Nematode Entozoa, certain Entomostraca, Aphides, and Bees; nevertheless the recent additions on this head are mainly a confirmation of the remarkable discoveries of Leuckart and Mecznikow in respect of the life-phases and development of Ascaris nigrovenosa. In this connexion one may particularize the observations of Prof. Leuckart respecting sexual dimorphism as it occurs in Coccus and in Chermes, of Prof. Hackel, who finds the naked-eyed Geryoniadce capable of producing (from the walls of the stomach) medusoids totally unlike their parents, and of Prof. Claus in respect of the Nematode Leptodera appendiculata. It was reserved, however, for Prof. Claparede to discover proofs of the existence of similar phenomena amongst the Annelids properly so called. By a recently published brochure (which the author has kindly sent me), I gather that the occurrence of two distinct sexual forms presented by Nereis dumerilii does something more than confirm the statements of the above-named authorities, since the dimorphic phases of this singular Annelid have something about them altogether peculiar, if not unique*. The sexually mature Nereis, we are told, loses for a time its sexuality, increases in size * Eecherches sur des Aimelides (p. 38). TirS des Arch, des Sci. de la Bibl Univ. Oct. 1869. |
