SOME ASCIDIANS FROM THE ISLE OP WIGHT. 135 



papillee 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 com- 

 pression and the presence of a pharyngo-cloacal slit. 



As to the former, it is a pure abnormality. By Professor Lankester's 

 kindness 1 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 pharyngo-cloacal slit, its presence in the adult and not in 

 the young may seem surprising, especially when its supposed morpho- 

 logical importance is taken into account ; but I have found exactly 

 the same phenomena in the species Asddiella 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 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 remarkable aperture is made 

 below (see p. 144). 



* 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 Professor Herdman in specimens of Asddiella aspersa from the 

 west coast of Ireland (Proc. Liv. Biol. Soc, v. 1891, p. 210, pi. x), an abnor- 

 mality which may also occur in Ascidia mentula, as I have myself observed in 

 a specimen from Lough Long. 



