BBKl'ISH ZOOPHYTES. 25 



those who have investigated the subject, and we may 

 conclude that chlorophyll is present in the protoplasm 

 of Hydra viridis as an animal and not a vegetable 

 product. The cells of both the ectoderm and endoderm 

 are vacuolated. This causes the granular appearance 

 seen under the microscope. The base of tbe hydra is 

 extended into a disc, the cells of which are to be 

 distinguished from the other ectodermal cells by their 

 cylindrical form, the possession of a highly refractive 

 fibrilla, and the presence of a similarly refractive mucous 

 secretion by means of which the animal is capable of 

 attaching itself temporarily to any object. Herr 

 Koi'otneff * proposes to call these elements "glandular 

 muscular.'"'' Some observers have described a narrow 

 canal or passage leading through the disc from the 

 body cavity for the passage of excrementitious matter,t 

 but the existence of this passage is denied by others 

 and is very doubtful. At the opposite end of the body 

 is the mouth or oral orifice (Plate I. fig. 1 m). In 

 many of the hydroids this mouth is placed at the end 

 of a pyramidal projection. Prof. Allman calls this 

 projection or proboscis the hijpostome : but, as this 

 proboscis does not possess any special function, it can 

 hardly be considered a separate organ. It appears to 

 be a mere modification of the shape of the distal end of 

 the hydroid. The mouth opens directly into the body 

 cavity {he) or intei^ior of the animal, which is hollow 

 throughout. Around the mouth of the hydra are a 

 row of tentacles, or prehensile organs, of great extensi- 

 bility, which are furuished to their very extremity with 

 thread cells. In the hydras proper, these tentacles are 



* Zool. Anzeig. iii. (1880), 454. 



t Gret-ne, " Manual of Ccelenterata," p. 22. 



