NOTE ON THE CILIATED PIT OF ASCIDIANS. 133 



(1) An anterior dorsal part (fig. 3 (1) ), exactly resembling 

 in structure the ganglion of the adult. 



(2) A mass (fig. 3 (2) ) lying ventral and posterior to (1), 

 composed of very large ganglion cells with very distinct nuclei 

 and nerve-fibres. 



(3) A nerve-cord (fig. 3 (3) ) passing off from (2) into the 

 tail. 



(4) A hollow sense-vesicle lying to one side of (2), and con- 

 sisting of a vesicle with thin anterior and thick posterior walls. 

 The unpaired eye is embedded in the wall at its antero-dorsal 

 angle, and the otolith is situated on its floor and projects 

 upwards into its cavity. This is the only part of the nervous 

 system which is hollow at this time. 



The ciliated pit opens into the buccal cavity and thence 

 passes back, lying ventral to the anterior part of the nervous 

 system, penetrates the junction between (1) and (2), and con- 

 tinues its course on the dorsal side of (2), ending blindly (E.) 

 not far from the atrial pore {A. P.). At two points it opens 

 into the solid nervous substance : first (B.), at about the 

 middle point of the ventral surface of (1); secondly (R-), on 

 the dorsal surface of (2). 



Maurice and Schulgin 1 failed to find the ciliated pit in the 

 embryo of Amarsecium, and state that there is no connection 

 between the buccal cavity and the nervous system. This con- 

 nection was quite clear in all my sections in which the 

 stomodseum and nervous system were definitely established. 



Kowalevsky 2 states that in Phallusia mammillata the 

 mouth communicates with the hollow anterior end (viz. sense- 

 vesicle) of the nervous system by a pore, which eventually 

 gives rise to the ciliated pit. It is possible that he may have 

 overlooked the existence of the ciliated pit in the embryo, as 

 in optical sections, with which he worked almost entirely, it 

 might appear as a simple pore. 



1 Th. Maurice and Schulgin, " Enibryogenie de PAmarcecium Proliferuni," 

 4 Annates des Sciences Naturelles," 1884. 



2 Kowalevsky, A., " Weitere Studien iiber die Eutwicklung der Einfachen 

 Ascidien," ' Arch, fur Mikr. Anat.,' 1871. 



