PTEROBRANCHIA OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 553 



their main axis. These are probably in part the result of the contraction of the zooid. 

 The aboral end of the testis may extend at least as far as the origin of the stalk from 

 the metasome. 



In some cases, though not in all, we have noticed the appearance indicated at the 

 aboral end of the testis in fig. 12. A tube, which seems to end blindly, extends into 

 the cavity of the organ as an invagination of the aboral end. The most probable inter- 

 pretation of this structure is that it is merely part of the wall of the testis which has 

 been invaginated, and that it has no special morphological significance. Its walls are 

 sometimes more pigmented than those of the rest of the testis, so that it may be a 

 conspicuous structure in entire preparations. 



When both testes are fully developed, as in fig. 5, t.l., t.r., they appear symmetric- 

 ally disposed on either side of the median dorsal mesentery of the metasome. The 

 central part of the testis is occupied by a mass of fully developed spermatozoa, while 

 there is a peripheral zone, of varying width, which contains very numerous nuclei, 

 indicating stages in spermatogenesis. In other cases, as in fig. 11, the two testes are 

 very difi"erent in size ; and in this particular case the left testis {t.l.) is fully developed, 

 while the right testis (t.r.) has remained vestigial. The figure shows the two trans- 

 verse genital mesenteries which run from the dorsal mesentery of the metasome, near 

 the region where the testes open to the exterior, to the two gonads. These transverse 

 mesenteries are no doubt the bearers of blood-vessels which pass from the dorsal vessel 

 to the gonads. 



Nervous System. 



We have no new observations of importance with regard to this part of the anatomy, 

 and it will be sufficient to note that the nervous system of C. agglutinans agrees in 

 essential respects with that of other species of the genus. 



Andersson has, however, stated (07, pp. 7, 8, 32) that while in the subgenus 

 Orthoecus the lateral nerves unite ventrally on the metasome to give rise to a nerve- 

 tract which has a single median thickening, Demiothecia difi'ers from it in having a 

 subsidiary thickening on each side of this median one. The occurrence of the three 

 nerve-tracts in the ventral region of the metasome and in the stalk was first pointed 

 out by Masterman (98, p. 513) in C. dodecalophus ; but it is not certain that the 

 character in question can be used to discriminate the subgenera of Cephalodiscus. 

 Thus in C. levinseni (Harmer, 05, p. 51, pi. xi. fig. 132), which belongs to Idiothecia, 

 a single nerve-tract is present in the stalk, while in C. nigrescens (Ridewood, 07\ p. 40, 

 text-fig. 15), belonging to the same subgenus, there are three nerve-tracts. Andersson 

 has, moreover, not sufiiciently taken into account the fact that in C. gracilis and 

 C. sibogse (Harmer, 05, pp. 52, 53), which belong to Demiothecia, only a single nerve- 

 tract is present ; while, on the other hand, C. dodeccdophu.<^, C. hodgsoni, and C. aequatus, 

 belonging to the same subgenus, have three nerve-tracts, C. agglutinans (fig. 13, n.t.) 

 agrees with the species of Orthoecus described by Andersson, and with C. {Idiothecia) 



