SIMPLE ASCIDIANS FROM AUSTRALIAN SEAS. 65 
The three known species belonging to the genus Poly- 
carpa show a good deal of. individual variation in the 
arrangement of the blood vessels forming the transverse 
vessels of the branchial sac, and also in the number of 
the stigmata in the meshes of the sac. The arrangement 
of the vessels forming the internal longitudinal bars of 
the sac is, however, generally supposed to be a constant 
character, but in one specimen of Polycarpa tinctor 
examined, only fowr internal longitudinal bars were 
present between each pair of folds of the sac, whereas the 
species has previously been described as possessing eight.* 
Perhaps one of the most interesting points in the 
anatomy of Ascidians is the ‘“‘ dorsal tubercle,” the careful 
examination of which, throughout the group, will help to 
throw, it is hoped, a little light on the question whether 
the dorsal tubercle is, or has been, anything more than 
merely the aperture of the duct from the neural gland, 
which is supposed to be the homologue of the pituitary 
body of the higher chordata. The dorsal tubercle is 
always liable to a considerable amount of individual 
variation. 
In Polycarpa tinctor, both horns, as the lips round the 
ends of the aperture are called, normally form spirals. 
In one specimen, however—the same which shewed a 
variation in the branchial sac—the left horn seems to 
have remained entirely uncoiled, and appeared almost as 
a straight line directed outwards from the side of the 
tubercle (Pl. VIIL., fig. 3). There are a good many speci- 
mens of this species in the collection, so we hope to examine 
the others and findif they are more normal in this respect. 
In a few of the undescribed species the dorsal tubercle 
has rather a remarkable form. This is especially the case 
* “ Challenger Reports,” vol. vi., pt. xvii., p. 170. 
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