PTEROBRANCHIA OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 547 



passes. Part of the coelomic opening {c.c.i.) is visible in the figure, while a line 

 crossing the epidermis just outside the canal indicates the commencement of the 

 external orifice. On the dorsal side of the section are visible the two testes {t.L, t.r.), 

 with the genital mesenteries passing from the median dorsal mesentery (d.m.^) of the 

 metasome to the gonads. The left testis (t.l.) of this individual is large and functional, 

 but the right testis (t.r.), which is cut close to its external opening, has remained 

 undeveloped. It appears in only a few sections of the series, and it does not anywhere 

 reach a size much larger than is shown in the section under consideration. The 

 asymmetry of the gonads is to be regarded as an individual peculiarity of this zooid 

 (see also p. 553). 



The demonstration of the number of arms, beyond the possibility of doubt, being 

 a matter of some importance, a plasticine reconstruction of this individual was made 

 according to the method described on p. 21 of the Siboga report (Harmer, 05). 

 A slight improvement was, however, introduced by drawing the sections on thin 

 drawing card ("Bristol board"). After cutting out the outlines drawn, the card was 

 covered with a layer of plasticine and was attached to the part of the model already 

 made. The cards representing the sections were left in situ in the model. The special 

 advantage of this mode of procedure is that the cut edges of the cards remain in the 

 model as a record of the outlines of the sections from which it has been constructed. 

 This obviates the danger of destroying the outlines of the sections when smoothing out 

 the intervals between two successive sections. If it should appear, moreover, that two 

 sections have not been fitted together in correct "registration," it is easy to separate 

 them at any time and to fit them together more accurately without losing the evidence 

 on which the model was built up. The study of this reconstructed model has enabled 

 us to obtain a clear idea of the position of the individual arms, of the parts of the 

 operculum, and of the arrangement of the gill-slits and collar-canals, with other features 

 of the zooid. 



Making use of this evidence, the correctness of which can be estimated from the 

 figures (figs. 11-6), the following description of the arms may be given. It will, of 

 course, be understood that their position in the individual under consideration is to a 

 large extent fortuitous, and that during life the arms could undoubtedly have assumed 

 many other positions. 



The first seven arms of both sides are directed dorsally, in line with the long axis 

 of the zooid. On the right side, however, the first arm passes at first posteriorly, at 

 right angles to the long axis, and then curves dorsally. On both sides the eighth and 

 ninth arms are in the main directed ventrally. The ninth right arm passes ventrally 

 from its origin, but soon curves across the ventral or anterior side of the zooid to assume 

 the position shown in fig. 11. The eighth arm begins by passing dorsally for a short 

 distance, and then doubles back sharply in a ventral direction. Fig. 10 represents the 

 arm at the point where this fiexure is taking place. It will be seen that the food- 

 groove faces posteriorly. The part which is marked R.^ represents the arm after the 



TRANS. ROy. SOC. EDIN., VOL. XLIX. PART III. (NO. 7). 72 



