THE VITAL PROPERTIES OF THE CELL 



165 



throughout the ectoderm. It consists of an oval capsule (a and 

 6), which is formed of a glistening substance, and which has an 

 opening in that end which is directed towards the external sur- 

 face. The internal surface is lined with a delicate lamella which, 

 at the edge of the opening, merges with the sheath of the cap- 

 sule ; the structure of this sheath is frequently very complicated 

 (cf. Fig. 70 a, b). In the figure, this sheath consists of a very 

 delicate filament and of a broad, conical, 

 proximal portion, which is situated in the 

 interior of the capsule, and is provided with 

 shorter and longer barbs. The filament 

 stretches from the end of the conical por- 

 tion, and is wound spirally round and 

 round it several times ; the free, internal 

 cavity is filled with an irritating secretion ; 

 the protoplasm, which borders on the ne- 

 matocyst, is differentiated to form a con- 

 tractile envelope, which also has an open- 

 ing to the exterior (Schneider V. 45). 



Near the opening of the capsule a rigid, 

 glistening, hair-like process, the cnidocil, 

 stretches out from the free surface of the 

 cell. If this is touched by any foreign 

 body, it communicates the stimulus to the 

 protoplasm. In consequence, the cnido- 

 blast, enclosing the nematocyst, contracts 

 suddenly and forcibly, thereby compressing 

 it, and forcing out the thread which is in the interior, so that it is 

 turned inside out, like the finger of a glove (Fig. 70 b). At first 

 the conical proximal portion is protruded with the barbs extended 

 outwards, next comes the delicate, rolled-up thread. The irritat- 

 ing secretion is apparently poured out through an opening in the 

 capsule. 



Some light is thrown upon the formation of this extraordinary 

 apparatus by the history of its development. First of all, an 

 oval secretion cavity is formed in the cnidoblast ; this cavity is 

 separated from the protoplasm by a delicate membrane, then a 

 delicate protoplasmic process grows into the secretion cavity from 

 the free end of the cell ; it gradually assumes the position and 

 form of the internal thread apparatus, separating upon its surface 

 the delicate enclosing membrane. Finally, the shining, tough, ex- 



Fig. 70. Thread cells of 

 a Cnidarian (from Lang ; 

 Hertwig, Zool., Fig. 161) : 

 a cell with cnidocil, and 

 the thread coiled up in the 

 capsule ; b thread evagi- 

 nated from the capsule, 

 and armed at its base with 

 barbs ; c prehensile cell of 

 a Ctenophore. 



