THE HYDRA. 09 



PLATE XVII. 



THE HYDEA. 



FIG. I. The green Hydra, at the maximum of contraction and elongation of its body. 

 The animal is drawn in the act of seizing a small Cypris, marked *. A. 2. 



FIG. II. The same, looked at from above. 



FIG. III. The above, with the hypostome everted. 



In this specimen two young tentacles were just appearing, the numerals i. to v. 

 indicate the order of development of these organs. Compare Jung (90). 



FIG. IV. Transverse section across the body of a Hydra, in the digestive cavity of 

 which a small crustacean was present. 



At * are seen the remnants of the digestible parts of that organism, not yet assimilated. 

 Hydra fusca. A. 4. 



FIG. V. Outline sketches of portions of two sections across the body of the same 

 animal, showing the variable extent to which the central cavity may be obliterated by the 

 amoeboid activity of the endoderm cells. 



At d. a diatom is seen, bodily ingested. H. fusca. A. 4. 



The most recent researches upon this intracellular digestion in the Hydrozoa, are 

 those of Laukester (95). A paper on the subject generally, by Mentschnikoff, will be 

 found in vol. xxiii. of the same journal. 



FIG. VI. A small portion of one of the tentacles, in the contracted condition. Surface 

 view. D. 2. 



FIG. VII. The same in elongation. 



In both these figures, only the nematocyst-beariug cell aggregates are shown. 



The small nematocysts, alone drawn in Fig. VII., are fixtures. 



FIG. VIII. The same, contracted under the influence of an irritant (1 p.c. acetic acid ) 

 The larger nematocysts are drawn as observed to become everted. They stain very 

 readily with magenta solution. D. 3. 



FIG. IX. The leading types of thread-cells, drawn after liberation from the body. 



i represents the functional, and ii the resting condition of the larger nematocysts ; in 

 i, the nucleated mass at the base and the fragment marked * represent the remains of the 

 parent cell. 



