764 PROCEEDINGS OF THE ACADEMY OF [DeC. , 



the body of the hydra. Below the mouth the hypostome become& 

 narrow aud tubular and distinct from the rest of the body, a decided 

 angle separating them at the level of the tentacles. 



Habits. 



One of the most striking habits of the adult jelly-fish is its pre- 

 hensile propensity. The adhesive organ at the "knee" of the 

 tentacle is composed of long slender glandular cells, packed into a 

 thick cushion which is inclosed within a strongly muscular rim or 

 collar (PI. XXXIII, fig, 20). This organ is located on the aboral 

 side of the tentacle. When at rest the jelly-fish lies on the bottom 

 with inverted bell, the tentacles widely extended horizontally and 

 attached to the bottom by means of the combined cement gland 

 and vacuum cup near the tip (PL XXXI, fig. 3). How this habit 

 of inverting itself could have come to be acquired primarily by the 

 adult medusa it is hard to see. But if, as I shall give my reasons 

 for believing, the medusa arises by direct metamorphosis from the 

 hydra, the habits of the hydra would naturally be more or less per- 

 manent in the adult. It may be that this particular habit is more 

 likely to be first acquired by the larva than by the adult. The 

 tentacles of the hydra reach a relative length greater than in the 

 case of any other known hydroid polyp. They frequently stretch 

 out in the water for a distance three or four times the height of 

 the polyp. Fig. 14, PI. XXXII, shows a hydra with the tentacles 

 fully extended, their tips touching. the ground in the characteristic 

 attitude. The drooping of the tentacles is evidently caused by their 

 extraordinary length, and is almost as unusual an occurrence among 

 the hydromedusre. At the points where the tips of the tentacles 

 come in contact with the bottom they spread out somewhat, forming 

 a sole-like surface which is closely applied to whatever object the 

 polyp is settled upon (PI. XXXII, fig. 14). This smearing out 

 of the tentacle tips is like that which occurs in live specimens of 

 hydra held between slide and cover-glass for examination. Both 

 polyp and medusa remain when at rest with the mouth expanded, 

 the manubrium stretching upward, the tentacles widely extended 

 and drooping to the bottom. When an animal swims against one of 

 the tentacles, the reactions are much the same in the polyp as in the 

 adult. The feeding habits of Gonionema have been described at 



