THE ANATOMY OF SCALIBKEGMA INFLATUM. 2'73 



a small mass near the base of the pajDilla, but these cells ai-e 

 rather smaller and more spherical than those described 

 above. 



At the base ot" the papillaj around the nervous axis, thei-e 

 are numerous very deeply staining nuclei-like bodies, about 

 4 — 5 fx in diameter. According to Eisig (1887, p. 505) these 

 are to be regarded as nuclei of multipolar ganglion cells 

 which have been deprived of their cellular substance. These 

 nuclei are situated upon a network of fine fibres, probably 

 nervous, since they are in close relation to the fibrils of the 

 branch of the spinal nerve supplying the sense orgau. 

 These fibres probably represent the protoplasmic part of the 

 cells of which the deeply staining bodies are the nuclei. 

 These and the basal ganglion cells are more numerous on 

 the ventral side of the axis of the organ than on tlie dorsal 

 side. This is probably accounted for by the fact that the 

 branch of the spinal nerve enters the papilla on the ventral 

 side. The nerve, soon after entering the sensor}^ elevation, 

 turns nearly through a right angle, and then runs along the 

 axis of the papilla, its ultimate branches terminating among 

 the ganglion cells and bases of the rods (fig. 30). 



Inserted into the base of the sensory papilla there is a 

 retractor muscle, the fibres of which spread out fanwise on 

 the basal part of the nervous substance of the sense 

 organ (figs. 16, 29). In some cases the muscle-fibrils extend 

 inwards into the papilla as far as the ganglion cells. In 

 favourable specimens the intimate relation of these muscle- 

 fibrils and the nerve-fibrils may be clearly seen, and it 

 appears probable that there is an arrangement similar to 

 that shown by Eisig for the Capitellida3 (1887, p. 505), viz. 

 that fine processes of the ganglion cells end in tlie fibrils of 

 the retractor muscle. The muscle is attached to the inner 

 end of the notopodial setal sac. 



The position of the sense organ between the two projecting 

 setal sacs affords it considerable protection, and additional 

 protection is given to the sensory area by its withdrawal into 

 a depression of the epidermis by means of the special 



