Dr. F. Leydig on Hydatina senta. 291 



seen in profile, it presents a tolerably strong convexity above. 

 To the single pit with the tuft of bristles two strong nerves 

 pass ; two other filaments also go to it ; these are of a muscular 

 nature, and I certainly saw them contract. These are the fila- 

 ments which Cohn describes as " going to the same spot in the 

 neck from other centres of the nervous system.^' The substance 

 of the fresh brain exhibits small nuclei, imbedded in a homo- 

 geneous fundamental mass, in which there are also still more 

 distinct molecules. The " large, circular, limpid vesicle, appa- 

 rently a vacuole, very frequently observed^' by Cohn, is, in my 

 opinion, a product of decomposition ; such appearances readily 

 appear in the delicate tissues of the lower animals, when their 

 vital activity is diminishing. 



I shall pass over the alimentary apparatus, as it has the same 

 structure which has been described by me in detail in other 

 Rotatoria, whose alimentary system is divided into pharynx, 

 oesophagus, stomach and intestine; only, with regard to the 

 pharynx, I may add, that the apparently beautiful cells, which 

 we think we see in the fleshy parts (see fig. 1, a), are the trans- 

 verse sections of muscles, and, indeed, of such muscles the sub- 

 stance of which is separated into a homogeneous cortical layer 

 and a granular axis ; this, therefore, is the reason why, on close 

 examination of these apparent cells, we perceive that they have 

 a clear, distinctly marked, peripheric layer, and internally a 

 granular mass of contents, from which a limpid nucleus glim- 

 mers*. 



With reference to the ^' respiratory system,'^ I am again under 

 the necessity of declining the corrections which Cohn has be- 

 stowed upon my statement. I had stated (/. c. sup.) that the 

 so-called tremulous organs [Zitterorgane) in their form repre- 

 sented two types, which, however, do not occur in one and the 

 same animal, but show themselves to be distributed in different 

 genera. Thus some remain as cylindrical tubes, of uniform 

 width, — such are possessed, for example, hy Notommata myrmeleo; 

 others are dilated at the free extremity, and thereby acquire a 

 somewhat trumpet-like form, as in Notommata centrura, Euchlanis 

 triquetra, Eosphora najas. Cohn, however, asserts, "that one 

 and the same tremulous organ presents one or the other form 

 according to its position," and in support of this, cites the figure 

 of Notommata centrura, in which both forms are to be seen. 

 But our author here evidently misunderstands the figures, for 



* In opposition to Cohn's statement that there is ciliary movement in 

 the oesophagus of Brachionus, I adhere to my previous assertion, that the 

 oesophagus of the Rotatoria is never clothed with cilia ; in Brachionus the 

 oesophagus is very short, and Cohn has erroneously transferred the strong 

 ciliation at the commencement of the stomach into the oesophagus. 



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