272 J. H. ASHWORl^H. 



The sense hairs form a dense tuft^ covering the flattened 

 area in the middle of the free surface of the papilla. They 

 are moderately stiff hairs, attaining" a length of about 40 — 

 50 jLi, but they are exceedingly delicate, being less than 1 /.« 

 thick at their bases. There are a hundred or more hairs in 

 each of the sense organs in the anterior and middle regions 

 of the body. The whole papilla is covered by the thin cuticle 

 continuous with that covering the general epidermis, but over 

 the hair field the cuticle is exceedingly thin, and is pierced 

 by the sense hairs. Beneath the cuticle, over the greater 

 part of the surface of the sensory papilla, there is a layer of 

 columnar or cubical epidermal cells, but in the hair field there 

 is a striking' departure from this arrangement. Here, below 

 the cuticle, are long, exceedingly thin columnar cells, closely 

 and regularly arranged. These rods are in most specimens 

 12 — 15 IX long (but in the largest specimen, 56 mm. long, 

 they attain a length of 20—25 fx) and about 1 jx wide. They 

 stain darkly, but not quite homogeneously, there being a 

 more deeply-staining, elongated, flattened nucleus near the 

 distal end of each rod. Each rod bears only one or two 

 hairs. The rods are continued inwards as delicate fibrils, 

 many of which maybe traced into continuity with the delicate 

 drawn-out ends of pyriform or fusiform ganglion cells, which 

 occupy the axis of the sensory papilla. Many of these gan- 

 glion cells are clearly bipolar, the outwardly directed process 

 being, as described above, in connection with the base of a 

 rod, the inward process passing into the nervous mass 

 formed by the spreading out of the spinal nerve in the basal 

 portion of the sensory outgrowth. In older specimens espe- 

 cially, the ganglion cells are nearly all obviously bipolar. 

 These ganglion cells are few in number, there being only 

 about eight or ten in each sense organ. They are usually 

 about 15 — 20 ju long, and about 8 — 10 fx wide, and their large 

 nuclei are 6 — 8 jx in diameter. Occasionally, especially in 

 large specimens, cells 30 i.l long with nuclei 8 — 12 fx in dia- 

 meter may be seen. 



There are other ganglion cells generally aggregated into 



