100 DR W. A. HERDMAN ON 



the red pigment spots at the branchial and atrial apertures and the pigment on 

 the aggregation of glands at the opening of the vas deferens are as bright and 

 conspicuous as is usual in the living animal. In one of the specimens preserved 

 in absolute alcohol, which was dissected, the inner surface of the test was found 

 to be closely ribbed longitudinally and less conspicuously so transversely. This 

 has been caused by the test having remained attached to the mantle during the 

 contraction of the latter, and having become impressed by the subjacent 

 strongly developed longitudinal muscles. 



The papillae at the angles of the meshes in the branchial sac seemed 

 larger than is usual in the species, and were certainly much larger than those 

 represented by Heller'" from a Mediterranean specimen. In some places 

 their length equalled the space between two neighbouring internal longi- 

 tudinal bars, so that when laid flat they stretched across the mesh. 



I have observed considerable individual variation in the branchial sac of 

 this species. In 188 It I noted a variability in the number of stigmata con- 

 tained in each mesh, and since then I have met with several other points in 

 which individuals differed. The specimens examined have been from various 

 parts of the British seas— the Firth of Forth on the east ; Lamlash Bay, Loch 

 Fyne, and the Sound of Mull on the west ; and Poole, Portland, and Dartmouth 

 on the south coast. I have also specimens from the Channel Islands, the 

 Chausey Archipelago, and the coast of Brittany, in addition to those collected 

 by the " Triton " in the North Atlantic. 



I have very rarely seen the arrangement figured by Heller | where the 

 meshes are represented as being greatly elongated transversely, and occupied 

 by two rows of extremely short stigmata. Usually the meshes are nearly 

 square, and are divided into two areas by a delicate transverse membrane, 

 which, however, does not generally interrupt the stigmata. This is shown at 

 tr' in fig. 2, where the membrane crosses the mesh, while the stigmata extend 

 behind. In the mesh below no transverse membrane is present, while 

 in fig. 1 three are seen, the central one being much the strongest. This 

 last arrangement was found to be very prevalent in the sac of the " Triton " 

 specimen examined. In some specimens the meshes, in place of being square, 

 are considerably elongated longitudinally — the reverse of the variation figured 

 by Heller — and the contained stigmata are very long and narrow. In this 

 case the meshes are always divided by from one to three transverse membranes. 



The papilhe upon the internal longitudinal bars appear liable to considerable 

 variations in their size and arrangement. In some cases they are present only 

 at the angles of the meshes, as shown in the lower part of fig. 2, and are then 

 all of much the same size. Where the meshes are divided there is usually a 



* Untersuchungen ilber die Tunicaten des adriatischen Meeres, Abth. ii. Taf. iv. fig. 6, Wien, 1875. 

 t Jour. Linn. Soc, Zool, vol. xv. p. 332. X Loc. cit. 



