422 DR S. F. HARMER AND DR W. G. RIDEWOOD ON THE 



bases, and are formed by the "Grenzmembran,"* are seen in this section. On the right 

 side the first five arms are similarly connected, but R, 6 and R. 7 are already free. A 

 comparison of this figure with fig. 8 will show that the first right arm has its axis directed 

 more or less horizontally (or backwards) at the base, and that the arm then curves 

 dorsally to assume the position in which it was seen in fig. 6. On the left side of 

 fig. 1 the lophophore is about to separate from the rest of the collar, and the anterior 

 dorsal horn of the collar-cavity is cut nearly at its tip. On the right side most of the 

 lophophore is separated, in the section, from the rest of the collar, but the first arm is 

 cut in a plane parallel to its food-groove, at its base. As the two sides of the groove 

 are continuous with one another in an adjacent section, the difference between the 

 two halves of the lophophore is thus not so great as it appears to be at first sight. 

 The food-grooves are obvious in all the arms except in L. c , where the groove has 

 opened out. 



Fig. 8, which is eight sections further on, shows the dorsal parts of the collar-cavity 

 (ft.c. 2 ) separated by a well-marked dorsal mesentery, at the ventral end of which lies the 

 notochord (nch.), followed by the pericardium (per.) containing the heart. Some of 

 the arm-grooves have disappeared. The eighth arm (L. 8 ) is visible on the left side, and 

 the ninth arm (L. 9 ) of the same side shows indications of joining the lophophore. The 

 relations of the first right arm (R. 1 ) have been explained in describing the previous 

 section. The sixth right arm has joined the lophophore, but the seventh arm of that 

 side is still distinct. 



Fig. 9 represents the tenth section from that shown in the preceding figure. The 

 eighth arm of the lef side is now separate and is cut longitudinally, while the ninth 

 arm of the same side is cut in a plane parallel to its long axis, so that its two sides are 

 completely separated by the food-grove. It will be noticed that this arm, the last of 

 the series, is closely related at its base to the opercular lobe (op.l.) of the same side. On 

 the right side, R. 7 has joined the lophophore ; R. s is beginning to appear, although still 

 separated from the lophophore ; and the tentacles of R. 9 are commencing to be visible. 



Fig. 10, which is four sections further on, shows the last two arms of both sides, 

 both ninth arms being connected with the corresponding opercular lobe at their base. 



In fig. 11, which is seventeen sections nearer the aboral end of the animal, the 

 eighth left arm has passed across the dorsal side to the right of the metasome, where 

 it lies close to the eighth right arm ; while the ninth right arm has passed across the 

 ventral side of -the zooid. The section passes through the region of the gill-slits and 

 collar-canals. Parts of the gill-slits (g.s.) are seen, in the region of their external 

 openings and of their origin from the pharynx (ph.), on both sides of the section. 

 The left collar-canal appears as a single cavity, but the right canal is cut twice, the 

 two parts being connected by the strong muscle of the canal. The reason for this 

 appearance is that the organ has a concave outline on its dorsal side, where the section 



* SPENGEL, J. W., "Die Enteropneusten d. Golfes v. Neapel," Fauna u. Flora d. Golfes v. Neapel, 18 Monogr., 

 1893, p. 452. 



(ROY. soc. EDIN. TRANS., VOL. XLIX., 546.) 



