680 ADDENDUM TO THE AUSTRALIAN HYDROMEDUSiE, 



regularity has been observed in no other species. It appears that 

 in this respect our Hydra is more highly developed than the 

 ■ others as the number of antimeres has been defined. The specific 

 name is derived from this chai-acteristic. 



The body is perfectly cylindrical, and extends under circum- 

 stances to a length of 15 mm., whereby it becomes about 0-1 mm. 

 thick. "When contracted it measures 1 x 0-5 mm. In the extended 

 state the body curves in a very graceful manner. The surface of 

 attachment is 0*4 mm. in diameter. The whole of the body is 

 colourless. Only the entodermal cells lining the narrow gastral 

 cavity have a slight yellowish tinge, visible only in transmitted 

 light. 



The arms when fully extended attain a length of 5 mm,, and a 

 thickness of 0-02 mm. They are usually curved backward in a 

 very graceful manner as shown on the plate. 



Histology. 



On the arms there are two kinds of cnidoblasts (fig. 2) with 

 different cnidocells. In this respect our species is very similar to 

 those Hydras which were examined by Jickeli, to whose figure 

 (Morphologisches Jahrbuch, Band VIII., Tafel XVII., 12), I 

 refer the reader. 



Elements like those described by Jickeli (I.e.) as ganglia cells 

 have been seen by me in the ectoderm of the tentacles of Hydra 

 hexactinella. They appeared more clearly visible after staining 

 with Beale's calamine than with haematoxylin, (Osmic acid in both 

 cases.) I have not been able to distinguish a nucleus in these 

 structures, the ganglionic nature of which appears to me a little 

 doubtful. I find (fig. 2 g) that these cells cause by their presence 

 the formation of a protuberance of ii-regular outline, being as they 

 are, interposed between the thin covering ectoderm and the 

 iinyielding supporting lamella below. I think it not quite 

 impossible that these large highly colourable bodies are the nuclei 

 of sensitive cells similar to the palpocils of Sarsia-polyps. 



I have not observed these structures in the fresh state. They 

 can only be studied with advantage in osmic acid specimens. 



