REPORT ON THE TUNICATA. 77 



angular appearance produced by the longitudinal ridges as described above. Tlie 

 appearance of these specimens is very different from that of the typical form, and they 

 might be regarded as a distinct variety. In one of the specimens collected on April 

 2G, 1876, however, the one end of the body was bifurcated while the other was j)ointed. 

 One of the specimens also collected at Station 222, in the Pacific, on March 16, 1875, 

 has the posterior end of the body bifurcated ; and of those collected in the North 

 Atlantic, on May 7, 1876, at night, a few have bifurcated ends, while the rest are 

 pointed. They are all rather prismatic in form. 



The very large specimen obtained in the South Pacific, on November 6, 1875, has 

 the posterior process of the body prismatic, and provided with slightly serrated ridges, 

 while the anterior process is plain. The equally large specimen obtained in the South 

 Pacific, on October 20, 1875, has the serrated ridges continued all along the body, but 

 they become more marked towards the posterior end. 



In other cases, again, the posterior process may be much reduced. In some of the 

 specimens collected in the South Atlantic, on March 10, 187G, the posterior process 

 may be said to be absent, and the body is rounded off close behind the nucleus ; while 

 in others from the same locality the posterior end of the body is well developed. 



The muscular system of the mantle is also liable to a certain amount of variation, 

 not in regard to the number and arrangement of the muscle bands, which seem to be 

 very constant, but in regard to the width of the bands and their distinctness. In some 

 cases, however, the musculature differs from that of the specimen figured by Traustedt. 

 Fig. 7 on PI. VI. shows a specimen obtained on November 6, 1875, in which the muscle 

 bands are relatively wide and distinct, and in which the firet muscle band (l) does not 

 reach so far forwards as usual, while the branchial muscle band (br.m.) extends farther back. 



In fig. 12 on the same plate a more important variation is shown. It represents 

 part of the musculature of a specimen obtained in the South Atlantic, on Fel)ruary 12, 

 1876, in which the fourth and fifth muscle bands not only touch at the sides of the 

 body, but actually join and anastomose, some of the fibres being traceable from the 

 fourth into the fifth, and others from the fifth into the fourth, so as to form a 

 decussation. In the same figure (PI. YI. fig. 12) emb. .shows the position of the embr\-o. 



The dorsal lamina ("gill") and dorsal tubercle of this species are shown about 

 twice the natural size in PI. VI. fig. 7, and more highly magnified in fig. 11. The 

 dorsal lamina increases gradually in width, and is simply marked with oblique bands. 

 The dorsal tubercle is of an elongated elliptical shape, with a narrow slit running along 

 the centre of its length. There is scarcely any projection, only the anterior end being 

 raised to form a slight hood (PI. VI. fig. 11). 



The nerve ganglion is small and rounded. It is placed immediately behind the 

 posterior end of the dorsal tubercle (PI. VI. fig. 11, n.g.) 



The solitary form does not show so wide a variation as the aggregated form in this 

 species, judging from the Challenger specimens. The largest complete specimen of the 



