THE HYDEOIDA. 



285 



DIAGRAMMATIC SECTION 

 OF HYDRA. 



close to the glass or weed by bending the body. It fixes itself by the tentacles, lets go the sucker 

 end, and remains for a s-acond or two, or more, head downwards. The original base moves forwards, 

 the body bends, and applies it to the supporting substance, to which it becomes adherent before the 

 tentacular extremity is set free. By this process of creeping some progress is made. But often the 

 Hydra gets as close to the top of the water as it can, and suddenly casts its body loose and turns the 

 base upwards, just beneath the surface. Waving the tentacles about, the 

 process of swimming is carried out, and the expansible disc, by floating on the 

 water, assists. 



After a while a little nodule appears on the body of the well-nourished 

 Hydra, and it grows outwards, and soon a crown of tentacles appears at 

 the free end. This is a bud which resembles the parent. A second may 

 gi-ow, and thus the stem and buds constitute a little colony. But the buds 

 drop off, and, having developed a base, become free, and take care of them- 

 selves. In some instances the Hydra diminishes in its girth at one spot, and 

 at last breaks off there. The free portion develops a base, and the fixed part a 

 crown of tentacles, and thus two individuals are formed by a process of 

 fissiparity. In the autumn, eggs escape from the outer tissues, having been 

 previously fertilised, and their central mass forms a clear ectoderm and a 

 darker endoderm. This escapes from its cover, and is set free. No cilia are 

 on it, and it gradually develops tentacles. 



. . ec, ectoderm ; en, endoderm ; in, 



Iii the adults the outer tissue, or ectoderm, of the body is continued ^t'-- : &S t )u l d taclt ' S: 6c> body " 

 up unto the tentacles, and consists of large nucleated cells, from whose bases 



filaments are continued inwards. Surrounding these neuro-muscular cells are others which contain 

 nematocysts. Moreover, minute points project from the surface cells. 



The inner tissue, or endoderm, which lines the visceral cavity and the inside of the tentacles, 

 contains cells, with amoeboid movement, and spaces in the midst called vacuoles. Some have long 

 cilia. The food passes down an opening in the midst of the bases of the tentacles, and reaches a 

 sac-like stomach, and particles of it get into the vacuoles, and are digested there. 



Vertical fibres and amoeboid cells exist between the layers of the body, or, rather, there is an 

 inter-cellular substance common to both layers. 



One of the most extraordinary gifts of the Hydra is its power of reparation of injuries, and re- 

 production of new individuals out of portions into which it has been accidentally or naturally divided. 

 If a tentacle be cut off, an entire animal is formed out of it ; if the body is cut in half, it will join 

 together again if the parts are placed together, and if not, two individuals will result ; if parts of one 

 individual are placed on the cut surface of another, they will grow together ; and if the body be turned 

 inside out, the old ectoderm takes on the digestive power, and the former endoderm that of the skin. 



Another common Hydra is the brown one (Hydra 

 fusca\ and its tentacles are longer than the body. 



These interesting and readily obtainable creatures are 

 species of a genus which belong to a family the Hydridae 

 of the sub-order Tubularia, classified under the great division 

 or order of the Hydroida. The Hydroida differ very con- 

 siderably from the other orders already noticed, in one 

 part or during the whole of their existence. They are very 

 plant-like and stationary during the whole of their exist- 

 ence, and they sometimes develop buds which become free- 

 swimming medusae. These reproduce ova, which become 

 The exceptions to this rule are few, and the characters of the Hydra 



NEMATOCYST OF HYDll V VIRIDIS. 



like the fixed or parent stock, 

 are rather exceptional. 



The fixed polypes of one of the sub-orders, the Trachomedusse, are not known, and they may not 

 have an alternation of generation ; and all the medusae of the plant-like or stationary forms have 

 not been discovered. Moreover, different genera of the Hydroida may have medusae, which present 

 the closest similarity, and the medusae alter much as they develop during growth. The polypes have 



