SENSORY CANALS OF THE COMMON SKATE. 101 



The Sensory Follicles or Pit Organs* 



The sensory follicles (" spalt-papillen " of Fritsch), as already indicated, lie in relation 

 to the lateral and infra-orbital canals. Those related to the lateral canal form a row 

 which extends from the region of the supra-scapula as far as the first dorsal fin — one for 

 every two segments. The follicles lie between the lateral line and the middle line of the 

 trunk and tail (p.o., fig. 6). 



The second group consists of two follicles (p.o. 1 , fig. 6) which lie in front of the audi- 

 tory pores ; while the third group consists of five follicles (p.o. 2 , fig. 6) which lie external 

 to the eye, immediately within the infra-orbital canal. The follicles though small are 

 quite visible without the assistance of a lens in fresh specimens, especially when the 

 epidermis is removed with the edge of a scalpel from the slight papillae by which they 

 open on the surface. With the help of a lens, the groove or split which runs across the 

 papilla, and leads into the pit, becomes evident. In those related to the lateral canal, and 

 the two in front of the auditory pores, the split is at right angles to the long axis of the 

 fish ; while in those lying within the infra-orbital canal, the split is nearly parallel to the 

 long axis. 



In uninjured specimens, each follicle is seen to present externally a slight rounded 

 projection, divided into two by a fissure which leads into the pit or follicle proper. 



The elevation consists chiefly of layers of epithelial cells, amongst which are many 

 goblet cells. In vertical sections the epithelial layer, still containing goblet cells, is seen 

 to extend well into the follicle. The bottom of the follicle is occupied by a large rounded 

 sense organ (fig. 10), which in many respects resembles a taste-bud. The sense organ 

 consists of pear-shaped sensory cells, with large oval nuclei and hair-like processes at the 

 outer end of each cell. The sensory cells are surrounded by columnar supporting cells, 

 in which the nuclei are deeper than in the hair cells. Passing to the sense organ of each 

 follicle are several nerve fibrils. These fibrils pass obliquely upwards through the 

 epidermic cells which underlie the follicle, aud terminate in the sense organ. The nerves 

 for the trunk follicles seem to come from the lateralis, those for the infra-orbital group 

 from the buccal, while those for the two follicles in front of the auditory pore may either 

 arise from the lateralis nerve or from the glossopharyngeal — this is a point we have not 

 been able to settle. We look upon these follicles as homologous with the pit organs of 

 Amia. Merkel states they have been found in Mustelus and Squatina. 



