NOTE ON THE CILIATED PIT OF ASCIDIANS. 135 



anterior dorsal part, this being rendered probable both by the 

 histological resemblance of the two structures, and also by the 

 fact that their position with regard to the ciliated pit is the 

 same in both cases. 



The adult condition most nearly resembling that of the 

 embryo Amaraecium is found in Clavellina. Here the ciliated 

 pit retains its connection with the nervous system, com- 

 municating with the ganglion in precisely the same way 

 as it communicates with the anterior part of the nervous 

 system in the embryo Amaraecium. In both cases it then 

 becomes narrower, passes on and ends blindly — the difference 

 being that in Clavellina it communicates on its ventral side 

 with the gland, in the Amaraecium embryo with the posterior 

 ventral portion of the nervous system. The relation of the 

 ciliated pit to the gland in Clavellina is identical with that of 

 the ciliated pit to the posterior ventral part of the nervous 

 system in the Amaraecium embryo. 



In the adult Amaraecium the position occupied in the 

 embryo by the posterior part of the brain and in Clavellina bv 

 the gland is filled with a mass of degenerated tissue, and the 

 communication between the ciliated pit and the nervous system 

 has been lost. In Ascidia and Ciona the degenerated tissue 

 is replaced by a mass of somewhat complicated glandular tissue 

 lying under the ventral wall of the ciliated pit and communi- 

 cating with it. 



In Phallusia mammillata the condition is still more 

 complicated, the gland communicating with the peribranchial 

 cavity by a number of secondary funnels, its opening into the 

 mouth being comparatively small. 



Suggestions as to the Functions of the Ciliated Pit. 



Judging from the fact that in the embryo Amaraecium the 

 ciliated pit is connected exclusively with the brain, it seems 

 probable that its original function was the aeration of the 

 brain; this mode of aeration being similar to that found in 

 Nemertines. It is doubtful whether it originally opened to 



