HYDRAHLE : HYDRA. 127 



from which a narrow canal is continued down to the sucker. When 

 contracted, and also when fully extended, the surface appears smooth 

 arid even, but " in its middle degree of extension," the sides seem to 

 be minutely crenulated an effect probably of a wrinkling of the 

 skin, although from this appearance Baker has concluded that the 

 Hydra is annulose, or made up of a number of rings capable of 

 being folded together or evolved ; and hence, in some measure, its 

 extraordinary ability of extending and contracting its parts.* That 

 this view of the Hydra's structure is erroneous, Trembley has 

 proved ; t and the explanation it afforded of the animal's con- 

 tractility was obviously unsatisfactory, for it was never pretended 

 that such an anatomy could be detected in the tentacula, which, 

 however, are equally or more contractile. These organs encircle the 

 mouth, and radiate in a star-like fashion ; but they seem to originate 

 a little under the lip, for the mouth is often protruded like a kind 

 of small snout : they are cylindrical, linear or very slightly tapered, 

 hollow and roughened, at short and regular intervals, with whorls of 

 tubercles which, under the microscope, form a very beautiful and 

 interesting object. According to Corda, each tentaculum forms a 

 slender membranaceous tube, filled with an albuminous nearly fluid 

 substance, intermixed with some oleaginous particles ; and at certain 

 definite places this substance swells out into tubercles or denser 

 wart-like nodules, which are arranged in a spiral line. (Fig. 29, a.) 

 Every nodule is furnished with several spinigerous vesicles, used as 

 organs of touch, and with a very singularly constructed organ for 

 catching the prey. The organ of touch consists of a fine sac enclosing 

 another with thicker parietes, and within this there is a small 

 cavity. From the point where the two sacs coalesce above, there 

 projects a long cilium or capillary spine, which is non- retractile, and 

 apparently immoveable. (c, d.) Surrounded by these cilia, and in 

 the centre of the nodule, is placed the captor organ, called the hasta. 



corpus naturaliter intestini instar cavum credideriin. Totum solidum et medullare, 

 pro admoto alimento, cerae instar, digitum admittentis, cavari concipio parenchyma et 

 alimentis insinuatis sese circumfundere. Qui alias per longitudinem dissecta Hydra, 

 illico qualibet portione deglutire, et cavo clauso alimenta condere posset? quod 

 tamen observare rarum non est." Elench. Zooph. 27, 28. For a view of the Hy- 

 dra's stomach, see Tremb. Mem. pi. 4, fig. 7, copied by Roget in his Bridgew. Treat, 

 ii. 74, fig. 241. 



* " The outward coat is white like the arms, and made up of minute annuli or 

 ringlets, that double in the midst, and can, occasionally, be folded close together, in 

 the manner of a paper lanthorn." Hist, of the Polype, 25. 



t Mem. 27. 



