26 



isolated sense-cells and nerve-endings in various parts of 

 its internal epithelium ; but is very badly provided with 

 more definite " sense organs." It has no true eyes — for 

 the little brightly-coloured dots or " ocelli" placed along 

 the margins of the apertures (PI. I.), and formed of a 

 group of modified ectoderm cells, supplied by a nerve and 

 imbedded in a mass of red and yellow pigment (PI. IV., 

 fig. 8), can scarcely be called such ; and it certainly has 

 no ears or otocysts. The tentacles are not very efficient 

 tactile organs, and the thin expanded margins of the 

 branchial and atrial siphons are apparently the most 

 sensitive parts of the body. 



But there is a curiously-curled projection, the Dorsal 

 Tubercle (PI. II., fig. 7, d.t.), placed at the front of the 

 dorsal lamina, in the prebranchial zone, near the entrance 

 to the branchial sac, which may possibly be an organ for 

 testing, by smell or taste, the quality of the water drawn 

 in through the branchial aperture. This organ has a 

 narrow slit which leads by means of a ciliated funnel into 

 a delicate non-ciliated tube, and this can be traced back 

 for a distance of several centimetres to a glandular mass, 

 the neural gland, formed of tubules lined by small cubical 

 cells, lying imbedded in the connective tissue immediately 

 underneath the brain (see n. gl., fig. 6, PI. II.). 



Hence, it has been suggested that the supposed olfactory 

 organ is merely the complicated opening of the duct from 

 the neural gland, and that this gland probably corresponds 

 to the hypophysis cerebri or pituitary body, which i_^s 

 found in all Vertebrata, from fishes up to man, attached 

 to the infundibulum on the lower surface of the brain. It 

 is probable that both views are partly right, and that 

 therefore the duct of the pituitary gland in the Ascidian, 

 opei]s into a sense organ placed on the roof of the mouth. 



Sensory cells have been found amongst the ciliated 



