82 



abandoned the vertical position altogether. What stimulus affects the organs it is 

 diflicult to imagine : and it is also ditficult to understand how the sensory hairs act 

 immersed as they are in the substance, whether it be mucus or not, which forms the 

 so-yalled cupula. But that these organs are of great importance to aquatic vertebrates 

 there can be no doubt, since they occur not only in all kinds of fishes but in aquatic 

 Amphibia : they are even present in the tadpole and other larval batrachians so long 

 as they retain their aquatic respiration. 



These sense-organs are in all cases first developed in the superficial epidermis ; the 

 development of the dermal tube in which they are enclosed takes place subsequently. 

 The dermal tube is formed from a superficial groove which appears on the surface of 

 the skin along the line where the sense-organs are situated. The groove becomes 

 deeper and its edges meet over it and coalesce everywhere except where the pores are 

 left by which the tube comniunicates with the exterior. In some fishes, e.g., Gobius, 

 the sense-organs remain superficial throughout life, no lateral dermal tube being 

 developed. 



As I have already mentioned (p. 76) there are also a number of superficial sense-organs 



on the under side of the head of the sole. These organs are situated in the depressions 



between the villi or tactile filaments with which the skin in this region is provided. 



The minute structure of this part of the skin is illustrated by Fig. I, Plate XV, 



which is drawn from a section of the part of the skin above the posterior half of the 



mouth cleft on the lower side of the body. Some of the filaments are long and slender, 



others short and blunt. Eunning up the centre of each filament is a sausage- shaped 



supporting rod which is composed of a tissue having the structure of fibro-cartilage. 



In section this body exhibits fibres anastomosing with one another and running in a 



direction transverse to the longer axis of the rod. These fibres contain nuclei, and 



the interspaces between them are filled by a homogeneous solid substance of the 



nature of cartilage. These supporting rods belong to the derma. The epidermis is 



continued over the filament, becoming thinner at the apex, and between the supporting 



rod or core and the epidermis is a continuation of the fibrous tissue of the derma. 



Eunning up the sides of the supporting rod are several fine nerves. These nerves send 



off fibrils which branch, and their ultimate ramifications enter the epidermis. There 



are no special sense-organs of any kind in connection with these nerve fibrils : the 



ultimate termination of the fibrils I have not been able to trace : they penetrate between 



the cells of the epidermis, and doubtless ultimately come into connection with some of 



the epidermic cells. Nerve-fibrils are known to enter the epidermis in the same way in 



the skin of the tip of the human finger, and in all probability help to give that skin its 



delicate sense of touch. I have indicated the connection of the ultimate nerve-fibrils 



with the epidermis on the right side of the figure. On the left side of the figure 



are sections of four short blunt filaments which do not project far beyond the surface 



of the skin, Around the cores of these, besides the black lines indicating parts of 



nerves, is a coarse stippling which represents the appearance of a curious granular 



