94 THE VOYAGE OF H.M.S. CHALLENGED. 



when seen in sections taken longitudinally through the wall. The two sphincters 

 are in fact merely local accumulations of these muscular rings, which are exceedingly 

 strongly developed (PI. XIII. fig. 2). The muscular rings rise much more than usual 

 above the surface of the wall, and seldom remain simple, but divide in transverse section 

 into two or three processes ; they may even divide repeatedly, and in this way give rise 

 in transverse section to the candelabra-like figure shown in Plate XIII. fig. 2. As the 

 upper sphincter lies at a little distance from the margin of the oral disk, it causes a deep 

 constriction of the wall and a corresponding external collar-like fold (PI. III. fig. 1, a). 



The twelve tentacles are placed on the margin of the oral disk where the latter turns 

 over into the wall ; they are pointed in the smaller specimens, obtuse and compressed in 

 the larger. They are distinguished in both cases by two longitudinal furrows, one of 

 which runs on the adaxial side (fig. 1, b), the other on the abaxial (fig. 1, a). The 

 muscular system is ectodermal, and tolerably thickly pleated ; it has the same char- 

 acter on the oral disk, which is so small that the tentacles appear to be placed immediately 

 on the oral margin. 



The oral opening (PL III. fig. 1, b) is circular and enclosed by twelve swellings. It 

 leads into a long similarly shaped oesophagus, which hangs down in the middle of the body; 

 the lumen of the oesophagus is narrow above and becomes wider below. I was not able 

 to find out oesophageal grooves, either in surface view after opening the animal (PL III. 

 fig. 1), or in transverse section (PL XIII. fig. 4) ; but, on the other hand, it is set with 

 twelve strongly-marked longitudinal ridges, corresponding to the insertions of the septa. 

 As the free margins of the longitudinal swellings are thickened, the intermediate furrows 

 are closed so as almost to form canals. 



The septa are thin, veil-like lamellae, with a thick longitudinal muscle which lies 

 much nearer the oesophagus than the wall. The muscle begins in the lower part, the 

 physa, bulges out in the region of the scapus, becomes narrow again in the upper part of 

 the body, and ends on the oral disk not far from the oral opening (PL III. fig. 1). In 

 transverse section it forms a muscular mass of considerable size, which rests like a cushion 

 on the surface of the septum, but is marked off from it by a deep groove which runs 

 beneath its margin on either side (PL XIII. fig. 4). As to its structure it is a thickly 

 pleated portion of the longitudinal muscular layer, which, moreover, forms a small 

 longitudinal cord close to the wall. The rudiments of a parietobasilar muscle are also 

 found in a similar position on the side of the weakly developed transverse muscles. 



The septa are placed at regular distances from one another, though at the same time they 

 are associated together in pairs. Two opposite pairs of septa have longitudinal muscles 

 on the sides turned from one another, whilst on either side there are two pairs with the 

 longitudinal muscles turned towards one another. This is, therefore, an arrangement of 

 the septa which must be taken as a starting point for all Hexactinise. Halcamjja clavtis 

 has only the principal septa, which, as we have already shown, are distinguished by a 



