179 
The giant cells and giant fibres of A. marina (fig. 54) 
have similar relations to those of A. grubs. As a rule 
the giant cells are almost mid-ventral in position, and their 
diameter in A. 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 fibrille, which 
branch; but the branches do not appear to reach the 
nucleus. These neuro-fibrille 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 
staming 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°65 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 papille of the proboscis, 
in the epithelium of most of which sense cells may be dis- 
tinguished ; (2) scattered sense cells in the epidermis, and 
(3) the notopodial sete, as Retzius has found nerve endings 
around their bases. Even when the animal is at rest the 
