524 COMPAEATIVE ANATOMY. 



be found in the Amphibia also. They are also found on the head 

 of Reptiles. 



2. Mucous canals. A system of tubes which branches regularly 

 in the head of Fishes, runs in the corium, and opens to the exterior 

 by branched canals, and at definite points. Near its opening the 

 tube contains the end-organ of a nerve-twig. A similar canal 

 extends from the head, along the side of the body, as far as the tail. 

 In the Ganoidei and Teleostei the nerve-endings of the system of 

 tubes are protected, both in the head and along this lateral line, 

 by an apparatus formed by the dermal skeleton, being embedded 

 either in modified scales, or being embedded in passages in the larger 

 covering bones of the head. The presence of goblet-shaped organs, 

 or similar structures along the lateral lines of the Amphibia (larval 

 forms and Perennibranchiata), points to a connection between these 

 organs and the " mucous canals " of Fishes. 



3. Grelatinous tubes. Thin-walled tubes of varying length, 

 filled with a gelatinous substance, open by fine pores, and carry at 

 the opposite end nerve-endings, which are placed in an ampulla- 

 like enlargement of varied form. These organs are found in 

 great quantity on the head of the Selachii, where they are generally 

 placed near the rostrum, but they are also found on more distant 

 parts ; thus, for example, in the Rays they extend as far as the 

 pectoral fins (Fig. 277, f). 



In the higher Vertebrata the nerve-endings in the integument are 

 less complicated ; as, for example, the corpuscula tactus in the papillaa 

 of the cutis, which are observed from the Amphibia onwards. 



Owing to the modifications undergone by different parts of the 

 body in consequence of the development in their integument of end- 

 organs of sensory nerves, special apparatuses are developed, which 

 function as tactile organs. The various arrangements of this kind 

 are wonderfully different, but as they are structures which owe their 

 origin to special adaptations, we can only mention them briefly. In 

 Fishes these organs are frequently represented by the "beards" 

 around the mouth, in which there are a number of goblet-shaped 

 organs. These are found in the Sturgeons, and many of the Cypri- 

 noids. In the Triglidte certain rays, which are separated off 

 from the thoracic fin, and are well supplied with nerves, function 

 principally as tactile organs. In Birds the sense of touch is not 

 unfrequently found in the soft tip of the beak ; this is the case in the 

 Rails, Ducks, etc. In the Mammalia, again, we find as tactile organs 

 stiff setiform hairs placed on the upper lip, or even above the eye ; 

 these hairs are not only greatly elongated, but are distinguished 

 from other hairs by the large supply of nerves to their follicles. 

 Finally, in many Mammals the limbs themselves, owing to the 

 rich supply of nerves on their volar and plantar surfaces, and to the 

 power of movement possessed by their terminal joints, have similar 

 functions. 



Leydig, Ueber Organe elnes sechsten Sinnes. N. A. Acad. Leop. Carol. Vol. XXXIV. 

 — JoBERT, Les organes da toucher. Ann. so. nat. Ser. V. Tom. XVI. 



