SPECIAL ORGANS. 87 



mentioned the Pyrophorus luminosus (cucujo), a large beetle 

 of Cuba ; Lampyris noctiluca (English glow-worm) ; J*hotinus 

 scintillans (small firefly, lightning bug); Photuris pennsyUanica 

 (large firefly) ; and Scolopendra eledrica (electrical centipede). 



XIII. 



SPECIAL OEGANS, 



A SPECIAL ORGAN is one subservient to a function ordina- 

 rily not essential to the economy of the individual. It may 

 be of a peculiar structure, or consist of modifications of a 

 kind already existing. Of the first an example is found in 

 ink-gland of Sepia (cuttlefish); of the second, in venom- 

 gland of Caudisona (rattlesnake). 



A special organ may have a sexual significance (see p. 102), 

 but more commonly is intended for purposes of offense and 

 defense. 



When a special organ is designed exclusively for reception 

 of external impressions, it is called a SENSE CAPSULE. In its 

 simplest expression a sense capsule is a chamber having con- 

 nection on one side with a nervous filament. It follows that 

 among the lowest animals no sense capsules are found. But 

 at the earliest point where differentiation of function occurs 

 and organ-making begins, they not infrequently attain to pro- 

 portio:is and degrees of development far in advance of other 

 apparatuses. Such special organs stand in exacter relation 

 to habit, and particularly to the medium in which the animal 

 lives, than to its zoological position. Thus in the higher Ce- 

 phalopoda the shelled Nautilus has an eye of much simpler con- 

 struction than its naked congener, the Sepia. In Talpa (mole), 

 living under ground, the eye is rudimentary or absent; but 

 its near associates, living above ground, have it fully devel- 

 oped. There is a power, however, apart from influence of 

 surroundings, which controls the production of sense capsules, 



