SPECIAL ORGANS. 89 



Echinodermata. In oral tentacles, ambulacra and peuicel- 

 lariae, the sense of touch partially resides. 



MOLLUSCA. The tentacles of Polyzoa and Brachiopoda, 

 the papillae on free edge of mantle of Lamellibranchiata, the 

 conspicuous cephalic tentacles of Gasteropoda, Pteropoda, 

 and Cephalopoda are doubtless of tactile significance. 



ARTICULATA. Organs of touch resident toward cephalic ex- 

 tremity. In Annelida processes of various forms and degrees 

 of complexity are observed. They are generally inarticu- 

 lated, though sometimes have distinct segments. In Crus- 

 tacea, M}'riapoda, and Insecta, in addition to palpi to certain 

 parts of mouth, jointed processes to cephalic segments 

 (antennae) are present. (See p. 41.) In Arachnida antennae 

 are absent. Palpi rarely absent, forficulate in certain scor- 

 pions, as Tlielyphonus and Phrynus. Anterior feet in latter 

 resemble antennae both in construction and use. 



VERTEBRATA. Pisces. The tumid lips of many fishes and 

 adjacent barbels are organs of touch. Special organs are in 

 addition met with in the integument. Placed within a canal 

 running beneath the 'lateral line' of osseous fishes, and ex- 

 tending thence to opercular and maxillary regions, are num- 

 bers of special sense organs. These consist of cellular struct- 

 ures appended to knob-like terminal filaments of the fifth or 

 tenth pair of nerves.* 



The Pacinian corpuscles are thought to be analogous to 

 these tactile organs. 



Batrachia and Reptilia. Special organs of touch absent. 

 Tongue generally protrusile and tactile. 



Aves. Sense of touch located in tongue and parts about 

 the bill. 



* Each knob is covered by a coat consisting of tiers of much elongated 

 cylindrical cells. Its substance consists of more or less gelatinous connect- 

 ive tissue. In the osseous fishes this nerve usually perforates the peculiarly 

 modified scale of the lateral line, which supports and encloses the canal at 

 these points. In the cartilaginous fishes, the canals have sometimes special 

 fibro-cartilaginous coats; or, if sacculi, a number of them may be contained 

 in a common cartilaginous investment, as in the Chimera. Leydig insists, 

 with great justice, on the identity of the structure of these organs with that 

 of the semicircular canals of the ear. (Huxley.) 



