SOME ASUIDIANS FROM THE ISLE OF WIGHT. 125 



opposite the cloacal aperture. It may therefore be admitted that 

 the presence of this slit is in some way a consequence of increased 

 size, and that its absence in young individuals is not a matter of 

 specific value. An attempt to explain the meaning of this remark- 

 able aperture is made below (see p. 132). 



II. 

 Ascidia depressa, Alder. 



AsciDiA. DEPEESSA, Alder. Cat. Moll. North. Durham, Trans. Tyneside Nat. Field 

 Club, 1848, p. 107. 



— — non Heller. Untersuch. iiber die Tunicaten d. Adriat, Meeres, 



Denks. d. Kais. Akad. Wiss. Wien., xxxiv, 

 ii, 1875, p. 15, Taf. v, figs. 10—12. 



— — nee Serdman. Notes on British Tunicata, Journ. Linn. Soc, 



XV, 1881, pp. 286, 287, pi. xviii, figs. 4, 5. 



— — nee Roule. Rech. s. les Ascidies Simples d. Cotes de Pi'ovence, 



Ann. Mus. d'Hist. Nat. Marseille, tom. ii, 1884. 



Under this name I describe a species of Ascidian of which I took 

 four specimens on May 11th. They were attached to the under sur- 

 face of a stone near the Zostera bed off Nodes Point. 



Specific diagnosis. — Body oblong ovate, much depressed, greenish when alive, 

 attached by the whole of the left side. Oral aperture subterminal; cloacal two thirds 

 of the way down, on the right side, near the dorsal edge. Test rather thin, carti- 

 laginous, provided with numerous minute tubercles on its free surface. Oral and 

 cloacal siphons, especially the cloacal, rather long. Stomach rounded, at the posterior 

 end of the body ; first bend of intestine considerably anterior to the cloacal siphon ; 

 rectum directed obliquely forwards, sometimes almost horizontal. Tentacles 25 to 30, 

 long and slender. Prxhranchial zone studded with minute papillae. Aperture of 

 dorsal tubercle horse-shoe shaped, horns not incurved, concavity anterior. Ganglion 

 much elongated, slightly dilated at each end. ^Epipharyngeal groove low, moderately 

 long. Dorsal lamina continued behind the oesophageal opening, fairly deep, strongly 

 ribbed on the convex side and regularly pectinated, with stout papillae profusely 

 scattered on the concave side. Pharyngeal ivall minutely plicated ; horizontal bars 

 usually broad and narrow alternately, their breadth never exceeding half the length 

 of the meshes ; internal longitudinal bars slender ; papillaj above the connecting ducts 

 erect, discoid, provided with a supporting ridge in front and behind ; no intermediate 

 papillae ; meshes square, each containing four or five stigmata. (Esophageal aperture 

 on dorsal side of pharynx, near its posterior end. 



The hodxj in all the specimens is much depressed, oblong in form, 

 with sloping and expanded edges, and attachment is affected by the 

 whole of the left side. The position of the oral and cloacal aper- 

 tures is indicated in fig. 7 (PI. VII), which represents the largest 

 individual of twice the natural size. The cloacal aperture varies 



