NOTES ON A TRACT OF MODIFIED ECTODERM. 103 
seems quite to correspond to the great ventral mass in 
Crama. In Lingula, besides the tracts of specialised 
tissue, individual epithelium cells may sometimes be seen 
to give off fibres which enter the cartilage, more especially 
in the region of the brachial muscle. 
These tracts of modified epithelium are apparently the 
same as those referred to by M. Joubin in Crania as being 
probably sensory in function, and possibly they correspond 
to the tracts of specialised tissue mentioned by some 
authors as occurring in some of the articulate Brachiopods 
and referred to as sense organs. The desirability, if not 
the necessity, of some means of testing the quality of the 
sea water, both for respiratory and nutritive purposes, 
is obvious, and the position of this modified ectodermal 
tissue on the delicate tentacles and the fold of the arms, 
in intimate connection with nerve fibres and cells, points 
I believe conclusively to its sensory nature. 
