268 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



fluid, but soon hardens on exposure to the air. The 

 silk thus secreted may be used as a kind of attaching 

 rope, as in some moths (Tortrix), or it may form an 

 investment for the larva, as is the case with the silk- 

 worm. In the larvae of the ant-lion (Myrmeleon) the 

 silk is secreted by the rectum, and escapes by a spin- 

 neret which is placed near the anus. 



A secretion of somewhat similar character is made 

 by some of the JLamelJibranchiata, where the 

 foot secretes a soft substance, which becomes hard and 

 chitinous on exposure to the air ; this 4 * foyssus " 

 may consist of threads fine enough to be woven into 

 gloves (Pinna), or of coarser filaments, as in the sea 

 mussel, or they may form firmer chitinous plates. 

 The function of these byssal threads, as may be well 

 seen in the Gloehidia, or young of the fresh- water 

 mussel, is one of attachment, 



Offensive organs of a somewhat similar character 

 are to be found in certain Holothurian. Con- 

 nected with and opening into the cloaca are a number 

 of tubes, compacted together into a more or less large 

 mass, and occupying in some cases a considerable 

 portion of the body cavity. The secretion of these 

 Cuvierian organs is expelled, on irritation, in the 

 form of fine tubes, which are capable of considerable 

 extension, and which also swell up in the water. 

 These expelled threads have a remarkable power of 

 adhering to any object which they may touch, and of 

 more or less completely entangling it and preventing 

 its escape. An English example of a Holothurian 

 thus provided is afforded by the so-called " Cotton- 

 spinner " (Holothuria nigra) ; the tubes are known to 

 have an irritating effect on the human skin. 



True electric organ* are developed in Torpedo 

 and other rays, in the eel (Gymnotus), and in the 

 teleostean Malapterurus. They are either placed in 

 the head (Torpedo), or in the tail (Gymnotus), or over 



