8 



member, Hydra Vulgaris {plate IV and V, H). The 

 hydra {of 0,2 to 3,5 Mm. length) is, besides the Cordy- 

 lopohora Lactjstris (in England), the only polyp found 

 in fresh waters. After water has stood a few minutes in 

 a glass, this yellowish polyp seeks to fasten itself by its 

 lower extremity, which terminates in a little disk or 

 sucker, and then gradually expends its four to ten tenta- 

 cles (feelers), waiting for food. If now any little animal- 

 cule comes in contact with one of its arms, they all con- 

 tract inwardly {plate IV, H, VIII.) and force the im- 

 prisoned animalcide into the mouth, which is placed in 

 the centre of the tentacles. There the nourishment is 

 brought into the alimentary canal (endoderm), and after 

 digestion is expelled through the mouth again. The ten- 

 tacles are furnished with so-called nettle-cells (lasso- 

 cells), {plate V at H, IX, and H X), which instantly 

 kill the animalcules as they come in contact with them. 

 Owing probably to the presence of free formic acid? 

 which is supposed to be contained in the projected end 

 of the nettle-twine (in the lasso-cell). They increase* 

 by sexual fructification (male and female ovules repro- 

 ductive calycle), by budding, and by separation (artifi- 



* James D. Dana gives the following different modes of reproduction : 



" I. Oviparous. 1. By ovules proceeding outward from the side of the 

 polyps, singly or in clusters. 2. By ovules formed from verticle lamellae in 

 the visceral cavity and ejected through the mouth. The viviparous is but 

 an accident in the oviparous mode ; the eggs within develope in the same 

 manner as internally and for like reasons, as the external waters have free 

 admission. 



" II. Gemmiparous. 1. By single buds, developing young, which after- 

 wards become free and independent animals. 2. By buds, which become 

 developed and remain persistent, and these may be either lateral or ter- 

 minal. 



" III. By artificial sections. This mode may depend on the same cause as 

 the general distribution of the budding function, and may be properly an 

 analogous process, but depending on the imperfect character of the nervous 

 system, or its absence." 



