OBSERVATIONS ON HYDRA 605 



a bowl-shaped space which stains less deeply than the normal 

 chitin (Text-fig. 3, 2). The thread can be seen lying in the soft 

 tissues, the first part at right angles to the external surface. 

 The distal part usually curves to one side. It is filled with 

 a darkly staining mass which is extruded either at the end or 

 along its course (Text-fig. 3, 3). In preparations of unfixed, 

 exploded nematocysts stained in methylene blue, the fluid 

 inside the penetrants stains deeply and can be seen partly 

 inside the capsule and partly in the form of tiny drops on the 

 outside of the thread. The thread has, apparently, rows of 

 minute pores or permeable areas through which the fluid can 

 escape, as well as by the opening at the end. 



The threads of the small pear-shaped nematocysts, or vol- 

 vents, when exploded, wind tightly round any protruding hairs 

 or bristles on the prey, and so hold it captive till it has been 

 killed by the poison of the penetrants. The thread is coiled 

 inside the capsule in two loops which lie on one another so that 

 they appear in section like one thick ring. There is a row of 

 small hairs on the thread arranged in a very open spiral. When 

 exploded, the thread coils up tightly, and in optical section 

 there is seen to be a narrow space along the axis of the coils 

 which is closely beset with hairs. This forms an efficient 

 mechanism for grasping the bristle, and there is some evidence 

 that the secretion in the capsule and thread, which stains deeply 

 with methylene blue, is sticky. Van Toppe (12) states that the 

 stimulus for the explosion of this type of nematocyst is different 

 from that exploding the penetrants, as the latter explode when 

 their cnidocils come in contact with flat surfaces, while the 

 former do not. 



The third and fourth types of nematocyst, the glutinants, 

 are usually^ cylindrical. One type is usually larger than the 

 other. In the former the thread shows four or five turns 

 almost at right angles to the long axis of the capsule and below 

 this is wound irregularly. In the latter the thread is wound 

 in an irregular figure of eight. Schulze (9) therefore calls 

 them streptoline and stereoline respectively. In some Hydras 

 (e.g. H. attenuata, van Toppe, H. circumcincta, 



