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XXVI. On Tomopteris onisciformis, EschsohoUz. By William B. Carpenter, Esq., 



M.D., F.E.S., F.L.S., F.G.S. 



Read Jan. 20th. 1859. 



1 HE animal which forms the subject of the present communication is one which has 

 not veiy frequently presented itself to the notice of zoological inquirers ; and it is not a 

 little singular that almost every one under whose observation it has fallen has given 

 an account of its structure differing in some essential particulars from that of other 

 observers. This circumstance may of course be readily accounted for on the hypothesis 

 that the several specimens examined have belonged not to the same, but to different 

 specific types. I cannot but believe, however, that a careful comparison of the pub- 

 lished accounts will lead others, as it has led myself, to the conclusion that all the speci- 

 mens described are referable to one and the same type, and that the differences are 

 chiefly those ot phase or stage of development. Por it is remarkable that the accordance 

 should be closest in those details of structure which might be expected, on the hypothesis 

 of specific difference, to be most liable to vary ; whilst the diversities are greatest iu those 

 features which seem most liable to undergo modification in the progress of development. 



I shaU first give a detailed account of my own observations, and shall then compare 

 them with the descriptions which I have since found to have been given of this creature 

 by those to whom it had already presented itself. My observations were made in the 

 month of September last, on specimens captured in Lamlash Bay, Arran. I was for- 

 tunate in being able at the time to obtaiu the assistance of Mr. George West, whose 

 intelligence and scrupulous accuracy as a microscopic draughtsman are well known to all 

 who have employed him ; and the drawings which accompany this communication haA-iag 

 been all executed by htm with the objects immediately before him, and under my own 

 direction as to the points to which his attention should be specially given, I cannot but 

 consider that they have a value much greater than can be attached to many of the repre- 

 sentations heretofore published, most of which seem to me to be little better than ideal 

 diagrams. 



The study of this animal, I may say in limine, presents peculiar difficulties, from its 

 incessant restlessness, and from the cu'cumstance that its delicacy is such, that confinement 

 is speedily fatal to it, its tissues and organs exhibiting a manifest tendency to disintegra- 

 tion some time before it ceases to move. We had, however, the great advantage of being 

 able to make our observations on several specimens in the younger and simpler stage which 

 I shall first describe ; and I feel sure that we had accurately mastered all the most important 

 features of its structure, before I met with the remarkable, and not a little perplexing form, 

 which I think I cannot be wrong in regarding as a more advanced stage of the same. 



VOL. XXII. 3 A 



