124 SOMK ASCIDIANS FROM THE ISLE OF WIGHT. 



but the lamina is a little deeper; the ribs are very strong and 

 regular, the teeth rather short, very regular, without intermediate 

 smaller ones ; the concave side of the lamina as described above. 



Behind the oesophageal aperture is a long smooth area (the " post- 

 buccal raphe " of Eoule), bounded on the left by a continuation of 

 the dorsal lamina, and on the right by a series of terminal elevations 

 of the horizontal membranes of that side, as in ^. mentula. 



Branchial apparatus. — This is much as in younger specimens, but 

 the arrangement of horizontal bars into primaries and secondaries, 

 &c., is less obvious, owing to the increase of the quaternary vessels 

 which are in many parts of the pharynx completely formed. Rudi- 

 mentary quaternaries are rare. 



The papillge at the junctions are bluntly conical ; the intermediate 

 papillae are well developed and slenderly conical. The meshes are 

 slightly longer than broad, except where new quaternaries are 

 forming, when they are twice as long. There are from five to seven 

 stigmata in a mesh. 



Between this Ascidian and the immature specimens of A. mollis 

 described above, the only points of difference, which are not 

 obviously the consequences of further growth, are the different plane 

 of compression and the presence of a pharyngo-cloacal slit. 



As to the former, it is a jDure abnormality. By Professor 

 Lankester's kindness I have had an opportunity this year of 

 examining in detail the collections of Tunicates in the Oxford 

 Museum, and I found there a specimen of Phallusia mammillata 

 which exemplified precisely the same kind of variation. The 

 broadly ovate test was compressed dorso-ventrally, the apertures and 

 ganglion being in the middle line of the upper side, and the viscera 

 and visceral septum of the test being correspondingly rotated. Yet 

 there were no structural differences at all to warrant a division of 

 the species. 



As to the phai-yngo-cloacal slit, its presence in the adult and not 

 in the young may seem surprising, especially when its supposed 

 morphological importance is taken into account ; but I have found 

 exactly the same phenomena in the species Ascidiella aspersa. The 

 ordinary specimens of that species show no trace of this aperture, 

 but I have seen a distinct pharyngo-cloacal slit in a particularly 

 large individual, taken from a Falmouth trawler, which I examined 

 this year at Plymouth ; in it the slit"^ occupied its usual position 

 * The walls of the slit were definite, straight, and smooth, resembling in all respects 

 those of the slit in Ascidia mentula. It must not be imagined that the slit, which I have 

 mentioned, was an irregular abnormality of the kind described by Pi-of. Herdman in 

 specimens of Ascidiella asjpersa from the west coast of Ireland (Proc. Li v. Biol. Soc, 

 V, 1891, p. 210, pi. x), an abnormality which may also occur in Ascidia mentula, as I have 

 myself observed in a specimen from Loch Long. 



