230 SUPPLEMENTARY NOTE. 



ratio to the stingiug powers of the animal. He believes, moreover, tliat experiment leads to the same 

 results, for he has allowed small liviug annelids and crustaceans to come in contact with the filaments 

 connected «itli the radiating septa of Actinia, and which are loaded with thread-cells, without finding 

 that the little animals so treated had thereby sufl'ered any harm. So also animalcules touched by the 

 filament ejected from the thread-cells in the tentacles of an Aciinia were not killed nor disabled by it. 

 Against the conclusions of Mr. Lewes there may be adduced the statements of other observers, 

 who describe tlie sudden arrest of all motion in the prey when touched by the thread-cell-bearing sur- 

 face ; and while it is true that the soft bodies of the annelids fit them for the experiment, it may be 

 objected to the employment of crustaceans that the resisting nature of their surface would enable them 

 to bear the exposure with impunity. Mr. Lewes, it is true, explains the arrest of motion in the cap- 

 tured prey by referring it to the habit of feigning death so well known in many insects and spiders, 

 but this can apply to only a very small number of the animals affected. That an actual penetration of 

 the thread-cell to a slight extent takes place I believe there is sufficient evidence, and the probability 

 that this is accompanied by the injection of a poison has been already maintained (see p. 129). Still, 

 however, with the investigations of so careful an observer as Mr. Lewes tending towards an opposite 

 conclusion, and supported as they are by those of jMobius and others, we cannot yet regard the alleged 

 stinging powers of the thread-cell as beyond doubt. 



EVOLUTION OF THE HYDROTDA— ANCESTRAL TYPE. 



Haeckel (' Generelle Morphologic,' vol. ii, p. li) regards the whole of the Ccehnterata as 

 derived fiom a hypothetical aboriginal form, to which he assigns the name of Archydra, and which 

 has continued itself with but little change down to our own times in Hydra and Cordylophora, the only 

 freshwater representatives of the Cwleiiterata. He supposes that these animals, in consequence of 

 the simple nature of the influences to which their fresh water habitat has exposed them in the struggle 

 for existence, have, like many other dwellers in fresh water, retained nearly their original simple 

 structure. With these persistent archydral forms he further unites those marine hydroids which come 

 nearest to Hydra in never giving origin to planoblasts, and in developing fixed sporosacs in place of 

 free locomotive buds. He regards Hydra as having for the Cmlenterata a significance similar to that 

 which Amphioxys has for the Vertebrata. 



Under the name of Protohydra Dr. Greef, of Bonn (' Zeitschr. f. wis. Zool.,' vol. xx, p. 37, pi. iv), 

 descrii)es a remarkable little organism which he obtained among diatom-mud and algaj from an oyster 

 bed at Ostend. It is almost of microscopic size, but has a very close relation with Hydra, from 

 which it chiefly differs in never developing a trace of tentacles, and in multiplying itself solely by 

 spontaneous fission. It is very contractile, varying in form from that of a sphere to that of an 

 elongated club. It attaches itself to algae and other bodies by one extremity, in the manner of the 

 Hydra, and is perforated at the opposite end by a mouth which leads into the cavity of the body. 



Its minute structure appears to be in all essential points similar to that of Hydra, and its body 

 is composed of a distinct endoderm and ectoderm with interposed muscular layer ; both endoderm and 

 ectoderm consisting of undoubted cells with nucleolated nuclei. The ectoderm contains abundance of 

 thread-cells, and the endoderm scattered reddish-brown pigment-granules. 



The endoderm may be broken down under the microscope, and the protoplasmic contents of the 

 cells liberated by the rupture of tiie cell-walls. The liberated protoplasm will then often show amoebi- 

 form movements, and emit pseudopodial prolongations. 



The spontaneous fission of the body takes place transversely, dividing the animal into quite 

 similar halves, each of which may again present successive fissions. 



Dr. Greef calls attention to a very interesting phenomenon which presents itself duiing the 

 division. While the constriction which is to result in the complete separation of the two segments is 

 as yet slight, the movements of the dividing animal are, as may be expected, quite simple, and such 

 as might be imagined to follow from the volition of a single individual ; but as soon as the constriction 

 has attained a certain depth the form of the segments has become changed, and each now carries out 

 in itself its own movements, but yet without the mutual dependence of the still united segments being 

 destroyed, for the movements of each segment are perfectly conformable to those of the other, and 

 buite svuchronous with them. If the one stretches itself out or contracts itself, so does the other, in 



