ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



377 



of the endoderm-cells. The study of sections shows that these 

 sensory cells are connected with the ganglion-cells, and the processes 

 which are given off from these ganglion-cells anastomose with each 

 other in such a way as to form a complete nerve-ring around the 

 mouth. This ring he regards as the central nervous system of 

 Hydroids, and he calls attention to the fact that it not only 

 originates from the endoderm, but is without a homologue in the 

 Medus8B, since none of the Medusae are known to have a nerve-ring in 

 this position. 



Development of the Tentacles of Hydra.* — The great variability 

 of fresh-water hydree demands that the order of development of the 

 tentacles should be tabulated in a great number of specimens, in order 

 to discover the law of their appearance. Jung has thus studied 

 nearly 250 specimens of three species ; and he concludes that, while 

 there is no fixed order, each species does have a typical or average 

 mode of development, which is more or less closely followed by the 

 majority. The law varies with the species, and the results of Jung's 

 researches are shown in the following diagrams : — 



Hydra viridis. 

 1. 



The vertical line is that axis of the bud which passes through the 

 axis of the parent. The upper series of diagrams shows the typical 

 order of appearance in normal buds of the three species named. This 

 order was followed in 46 per cent, of 156 specimens of H. grisea,in 

 83 per cent, of seven specimens of H. oligactis, and in 55 per cent, of 

 21 specimens of H. viridis. The second line shows the order of 

 re-appearance in specimens after cutting off the oral end of the body 

 with the tentacles. It was followed in 69 per cent, of 48 specimens 

 of H. grisea, in 3 specimens of H. oligactis, and in 47 per cent, of 

 12 specimens of H. viridis. 



* Morph. Jahrb., viii, (1882) pp. 339-50. Cf. Science, i. (1883) p. 81. 



