450 PROF. W. A. HER OMAN ON BRITISH TUNICATA. 



inner surface of tlie raautle. In some cases a good deal of opaque 

 white pigment present. 



Branchial sac witli four folds on eacli side. The two dorsalinost 

 folds larger than the others. There are from 6 to 12 bars on a 

 fold and 3 or 4 in the interspace between two folds. The mesbes 

 are always elongated transversely, and contain from 6 to 20 long 

 narrow stigmata. The meshes are generally divided by a delicate 

 horizontal membrane. The transverse vessels are of three sizes, 

 regularly arranged. 



Endostijle large and conspicuous. 



Dorsal lamina is a plain membrane with transverse ribs but 

 no teeth upon the margin. 



Tentacles simple, 20 to 30 in number, rather large and stout, 

 alternately larger and smaller, sometimes with a number of very 

 small ones in addition. 



Dorsal tulercle prominent, large, and simple, with the aper- 

 ture turned to the left side and the horns slightly turned in. 



The alimentary canal is large ; the stomach is long and is 

 longitudinally folded. 



The gonads consist of a dorso-ventrally running undulating tube 

 with four or five branches directed anteriorly (Plate XXXVI. 

 fig. 1). The duct runs posteriorly from near the dorsal end of 

 the main tube. There are a number of endocarps scattered 

 around the gonads and between their branches. This condition of 

 the reproductive organs is very different in appearance from that 

 of Styela monoceros (see Plate XXXVI. figs. 1 and 2), and by this 

 character the two species can be distinguished at a glance when 

 the mantle has been cut open and its inner surface exposed. 



Sttela monoceros (Mdller). (PI. XXXVI. fig. 2.) 

 External appearance. Elongate elliptical, not compressed 

 laterally, attached by the base and^ a little way up the ventral 

 side. The anterior end is marked by a curious spine-like 

 projection composed of a solid outgrowth of test ; it is situated 

 midway between the branchial and atrial apertures. The 

 branchial aperture is rather prominent and conspicuous, almost 

 terminal, but inclining a little more towards the ventral edge ; 

 the atrial is smaller and less conspicuous, placed a little way 

 down the dorsal side. The surface is considerably creased and 

 roughened, in some specimens a good deal covered with zoophytes, 

 shells, and other foreign matter. Colour dirty greyish yeiluw. 



