132 JOURNAL OF THE [Ju^Yj 



usually collected into reniform groups, or knobs. They are, 

 however, found, in various stages of development, scattered 

 through the deeper tissues and in widely separated parts of the 

 animal. When seen before excitation has caused the ejection 

 of the thread, they consist of a thin-walled sack, either globular 

 in form or of an elongated egg-shape, within which is coiled a 

 long and elastic filament, and which is probably filled by a fluid. 



These cells vary greatly in size, in different orders and genera. 

 In the " Ruby Medusa," Tiirris Neglecta, Mr. Philip H. Gosse 

 estimated their length at not over the atiVs of an inch, while in 

 the " Red-lined Medusa," Chrysaora cyclonota, he says " the 

 largest were about ^^riJtf inch in length, the smallest about siros 

 inch, with the thread occupying an oval cavity about two-thirds 

 of the entire volume." In the tentacle of the "Portuguese 

 Man-of-war," Physalia pelagica, which 1 have under one of my 

 microscopes this evening, the cells average about ^3(7?r of an inch 

 in diameter, and are perfectly circular in outline; but in the 

 " Bermuda Madrepore," Isophyllia dtpsacea, of which I have a 

 preparation under another microscope, they are all of an oval 

 form, and, in the largest kind, attain to the very unusual magni- 

 tude of about ^2^th of an inch in the longest direction, by 

 about one-quarter to one-fifth of that measurement at the widest 

 part of the short diameter. In some of the Hydrozoa, however, 

 and in most of the Actinozoa several entirely different sorts of 

 thread-cells occur in the same individual. Thus, in my speci- 

 men from Isophyllia, I find three quite distinct forms, the largest 

 of which is represented in the drawings. These forms are so 

 unlike in size and construction that it seems beyond doubt that 

 they serve dissimilar purposes in the economy of the Madrepore ; 

 but I am unable to say what position they respectively occupied 

 in the living creature. 



By far the larger part of all recorded knowledge of thread- 

 cells has been derived from Mr. Gosse's observations, made 

 some twenty-five or thirty years ago. He classifies these organs 

 mainly according to the manner in which the filament is dis- 

 posed within the capsule. Thus, he describes chambered, tangled 

 and spiral cnidse. But, in the three kinds observable in the 

 specimen now under the microscope, the differences extend to 

 the size and shape of the capsule, to the presence or absence of 

 a smaller capsule within the larger, to the way in which the thread 



