38 THE VOYAGE OF H.M.S. CHALLENGER. 



TJtc Dorsal Lamina (Oral Band, Ventral Plait, Epibranchial Fold, Languets). 



This structure occupies the region of the branchial sac directly opposite to the 

 cndostyle, and marks the dorsal edge. 



In the majority of Simple Ascidians it is present in the form of a membrane of 

 varying breadth, according to the species, which commences anteriorly by the union oi 

 two pads which converge from the dorsal ends of the posterior peripharyngeal band, and 

 are separated by a groove, called by Julin the " epibranchial groove." This groove is also 

 of varying length, according to the species. In some (e.g., Corella parallelogra/mrna) it 

 is comparatively short, while in others (e.g., Ascidia venosa) it is of considerable length. 

 Although it frequently approaches close to the two halves of the peripharyngeal groove, 

 it appears to be always completely shut off from both by the union of the two sides of 

 the posterior peripharyngeal band. It is only this first or anterior portion of the dorsal 

 lamina which is united to the mantle ; the rest of it is free, the branchial sac being in 

 this region separated from the mantle by the dorsal part of the peribronchial cavity. The 

 epibranchial groove is lined in its entire extent with low columnar ciliated epithelium. 

 Behind the epibranchial groove the dorsal lamina usually widens gradually as it approaches 

 the oesophageal aperture, and then narrows suddenly, and terminates in a low ridge lead- 

 ing round the posterior end of the sac to join the extremity of the endostyle. 



The dorsal lamina has, especially in its posterior broader part, the free edge somewhat 

 bent round, usually to the right but sometimes to the left side (fig. 9, p. 40), so as to 

 form a semi-canal leading down to the oesophageal aperture, and it is along this groove 

 that the string of food- particles agglutinated with mucus passes on its way to the 

 oesophagus. The left (convex) side 1 of the lamina is in many species marked by a series 

 of more or less distinct transverse ridges or ribs, usually corresponding to the transverse 

 vessels and the horizontal membranes of the branchial sac, and crossing from the base to 

 the free edge, where they frequently terminate in projecting teeth or knobs (PI. XXXI. 

 fig. 7). 



In different species the dorsal lamina differs considerably in its breadth and the 

 presence and amount of development of ribs and teeth— the length depends upon the 

 position of the oesophageal aperture. 



In several genera of the Ascidiidse (dona, Corella, Abyssascidia, Chelyosoma, and 

 Corynascidia) the dorsal amina is represented by a single, or in some cases a double, 

 series of triangular flaps or tapering finger-like processes named languets (PI. XXV. 

 fig. 7, PI. XXVI. fig. 7, and PI. XXXIV. fig. 9). These are also found in the 

 Clavelinida3 and in some of the Cynthiidae (e.g., Cynthia and rarely Styela). In some 

 cases the languets are united together at their bases by a narrow membrane, so as 



1 Sometimes Loth sides are ribbed transversely. 



