44 



THE VOYAGE OF H.M.S. CHALLENGER. 



the duct and open into it in its posterior half or three-fourths, the glandular epithelium 

 of the tubes being continuous with the epithelium of the duct. 



A.S to the function of the neural gland, van Beneden's recent suggestion that it is a 

 renal organ in connection with the nervous centre seems probable, and is supported by 

 the cisr of Ascidia mammdlala, where the apertures of the numerous secondary ducts or 

 infundibula open into the peribranchial space. 



The duct (figs. 10 and 11, gl.d.) is delicate, and is lined by a single hvrer of cubical 

 epithelium. It runs anteriorly, directly under the front part of the nerve ganglion 



Tjrn 



. at.iL 



Fig. 11. — Diagrammatic longitudinal vertical section through the dorsal region of the anterior end of a Simple Ascidian. 



hr.iK. branchial nerve ; at.n., atrial nerve ; n.g., nerve ganglion ; gl., subneural gland ; gl.d., duct of the gland ; d.t., dorsal tubercle ; 



p.p., peripharyngeal band ; e.g., epibranchial groove ; d.L, dorsal lamina ; Br.s., branchial sac. 



and above the anterior part of the dorsal lamina, till it terminates by a complicated 

 aperture best known as the olfactory or dorsal tubercle (fig. 11, d.t.), situated in the 

 dorsal region of the anterior end of the branchial sac. 



The Dorsal Tubercle (The Anterior Tubercle, the Branchial Tubercle, the Ciliated Organ, 



the Olfactory Tubercle.) 



This, the aperture of the duct from the neural gland, is situated in a diverticulum 

 from the prsebranchial zone, the peritubercular area, formed by the bending jwsteriorly 

 of the right and left peripharyngeal bands before they join at the anterior end of the 

 dorsal lamina. Primitively, there is little doubt, it was in the form of a simple circular 

 opening probably with prominent edges. It is still found in this or a very slightly 

 modified form in Molgula pyriformis and Eugyra kerguelenensis. In most cases, how- 

 ever, the aperture is found in a much more complicated condition. This seems to have 

 resulted from a forcing backwards of the anterior part of the edge till it almost came 

 in contact with the posterior part, thus reducing the circular aperture to a slit curved in 

 the form of a semicircle, with its concavity directed forwards, and bounded by prominent 

 lips, usually more conspicuous than the aperture. This stage is found in Ascidia seabra. 



Further complications are produced by the lips round the ends of the slit or " horns," 

 as they may be called, being greatly prolonged and bent or coiled in various directions. 



