SEA-FISHERIES LABORATORY. 259 



Tlie giant cells and giant fibres of A. marina (fig. 54) 

 have similar relations to those of J.. grubU. As a rule 

 the giant cells are almost mid-ventral in position, and their 

 diameter in ^4. marina is from about 04 to '08 mm. The 

 cell is pyriform in shape, and the narrow extremity is 

 prolonged upwards, and follows the course described 

 above. Each cell has a fibrillar sheath. The protoplasm 

 is clear, but in favourable preparations is seen to be 

 traversed by delicate, darkly-staining fibrillse, which 

 branch ; but the branches do not appear to reach the 

 nucleus. These neuro-fibrillse may be traced into the 

 process of the cell, and for some distance towards the 

 giant fibre, with which the process is connected. The 

 nucleus is a large vesicular structure, with a diameter 

 about one-third that of the cell to which it belongs ; 

 within the well-marked nuclear membrane is a small 

 amount of chromatin reticulum, and usually one deeply 

 staining nucleolus. The nucleus is nearly always 

 excentric, being placed near the broader end of the cell. 



The giant cells are not differentiated in a post-larval 

 specimen 4"5 mm. long. In specimens 17'5 mm. long and 

 upwards they are well marked, but they are apparently 

 no larger in specimens 250 mm. long than in others one- 

 fourth this length. 



Sense Organs. 



The sense organs are the otocysts, the nuchal organ, 

 the eyes and the prostomium. To this list of sensory 

 structures should be added (1) the papilla? of the proboscis, 

 in the epithelium of most of which sense cells may be dis- 

 tinguished; (2) scattered sense cells in the epidermis, and 

 (?)) the notopodial setse, as Eetzius has found nerve endings 

 around their bases. Even when the animal is at rest the 



