GENERAL BIOLOGY. , 25 



Pigs. 



hypo 



cells: .,„ , — p ^--- --- - . . ., 



porting lamella; cl, chlorophyl-forming Dodies; c, ciliam. 

 Pig. 41.— The green hydra, at tne maximum of contraction and elongation of its body. 



The creature is represented in the act of seizing a small crustacean (A, 8). 

 Fig. 42.— Transverse section across the body of a hydra, in the digestive cavity of 



which a small crustacean is represented. 

 Fig. 43.— The leading types of thread-cells, after liberation from the body (F, 3). The 



cells are represented in the active and the resting conditions; in the former all the 



parts are more distinctly seen in consequence of the necessary eversion. 

 Fig 44. — Small portion of a transverse section across the body of a green hydra 



(D, 3). 

 Fig. 4S.— a large brown hydra bearing at the same time buds produced asexually and 



sexual organs. 

 Fig. 46. — Larger cells of the ectoderm isolated. Note the processes of the cells or 



Kleinenberg's fibers (F, 3). 

 All the cuts on pages 9 to 34 have been selected from Howes' Atlas of Biology. 



cylindrical form ; provided at the oral extremity with thread- 

 like tenacles of considerable length, which are slowly moved 

 about in all directions; but they and the entire body may short- 

 en rapidly into a globular mass. They are usually attached at 

 the opposite (aboral) pole to some object, but may float free, or 

 slowly crawl from place to place. It may be observed, under 

 the microscope, that the tenacles now and then embrace some 

 living object, convey it toward an opening (mouth) near their 

 base, from which, from time to time, refuse material is cast out. 

 It may be noticed, too, that a living object within the touch of 

 these tenacles soon loses the power to struggle, which is owing 

 to the peculiar cells {nettle-cells, urticating capsules, nemdto- 

 eysts) with which they are abundantly provided, and which se- 

 crete a poisonous fluid that paralyzes prey. 



The mouth leads into a simple cavity (coelom) in which 

 digestion proceeds. The green color in Hydra viridis, and the 

 red color of Hydra fusca, is owing to the presence of chlorophyl, 

 the function of which is not known. Hydra is structurally a 

 sac, made up of two layers of cells, an outer (ectoderm) and 

 an inner (endoderm) ■. the tentacles being repetitions of the 

 scructure of the main body of the animal, and so hollow and 

 composed of two cell layers. Speaking generally, the outer 

 layer is devoted to obtaining information of the surroundings ; 

 the inner to the work of preparing nutriment, and probably, 

 also, discharging waste matters, in which latter assistance is 

 also received from the outer layer. As digestion takes place 

 largely within the cells themselves, or is intracellular, we are 

 reminded of Vorticella and still more of Amoeba. There is in 

 Hydra a general advance in development, but not very much 

 individual cell'specialization. That of the urticating capsules is 

 one of the best examples of such specialization in this creature. 



