ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 409 



He distinguishes four species, noting the diagnoses and synonyms, viz. 

 Hydra viridis, H. grisea, H. fusca, and less certainly H. attenuata of a pale 

 straw-yellow colour. A careful and detailed account is tLen given of the 

 histology of this organism which has been the subject of so many investi- 

 gations. From the nature of the case the results are rather corroboratory 

 than novel. 



The second part of the memoir is devoted to a description of the 

 author's numerous experiments on the familiar power of regeneration and 

 wound-healing exhibited by mutilated polyps. Slices were cut out of the 

 body in any direction, and divided into four or so portions, which regene- 

 rated new organisms. As has been previously noticed, the experiments of 

 Trembley as to the survival of Hydra after being turned inside out are 

 confirmed, while it is further shown that there is no modification of ecto- 

 derm into endoderm, and endoderm into ectoderm, but that new growths 

 restore the old layers. An ectoderm grows over the exterior, and the 

 elements, no longer able to continue in their reversed conditions, are 

 absorbed and replaced by fresh cells. The concluding chapter of Nuss- 

 baum's paper is an interesting historical sketch. 



Nematocysts of Hydra fusca.* — Mr. E. J. Harvey Gibson, by the use 

 of eosin and some specially large Hydrse, has been able to make out dis- 

 tinctly the anatomy of the resting stage of nematocysts ; the wall is firm, 

 transparent, and more or less elastic; it is occasionally surrounded by a 

 clear crescentic space, which intervenes between the capsule and the proto- 

 plasm of the ectoderm-cell ; at the narrower end of the capsule a distinct 

 depression can, under a high power (Zeiss 1/12 in. oil-immersion), be made 

 out ; on one side of this rim there is an appearance of discontinuity in 

 the substance of the capsule. In the interior of the capsule there is a 

 funnel-shaped membranous tube which at about one-third of the length of 

 the capsule becomes enveloped in a general mass which occupies the rest 

 of the space. Mr. Gibson calls this tube the "pharynx." When a living 

 Hydra is " irrigated " with dilute acetic acid, tlie pharynx and an enormously 

 long thread are everted : to investigate the stages of eversion a living tentacle 

 was stimulated with acetic acid, and this was immediately followed by 

 the application of a small drop of one-quarter per cent, osmic acid. Most 

 of the nematocysts were arrested in the act of exploding ; some had only 

 the pharynx evaginated ; some, in addition, had a thread about four times 

 the length of the capsule protruding, and that thread was double the usual 

 thickness of the entirely everted thread ; in those in which the evagination 

 of the thread had been arrested in its last stage there was a long club- 

 shaped extremity ; in most of these and also in the partially everted threads, 

 careful focusing revealed another thread distinguished by its faint spiral 

 twist. The author is of opinion that, though the forces which bring about 

 the evagination of a nematocyst may be physical, they are under the 

 command of the Hydra; the initial act in the process is the dissolution of 

 continuity between the lid and the capsule. 



The development of the nematocyst commences witli a granular dif- 

 ferentiation of the protoplasm in any part of the cell ; the granule grows 

 and becomes a circular sac ; at one point an invagination is made, and 

 the coiled finger-like process grows and coils round and round the central 

 tube ; the capsule broadens and the central pharynx developes the arrow- 

 head spikes of the adult nematocyst. 



Living parasitically was a species of Eiiplotes which wanders freely all 

 over the Hydra ; they were found to contain many nematocysts, both large 



* Proc. Lit. and Philos. Soc. Liverpool, xxxix. (1885) pp. 29-38 (1 pL). 

 1887. 2 E 



