118 THE VOYAGE OF H.M.S. CHALLENGER. 



and fills the greater part of the umbrella cavity, its oral margin (am) reaches to the 

 opening of the latter. Its typical form is a regular quadrangle prism, whose lower 

 surface occupies the simple cmadrate oral opening, whilst the upper surface is formed by 

 the endodermal surface of the central gelatinous umbrella. Both these two surfaces and 

 each horizontal transverse section of the oesophagus describe a regular cross, as four 

 perradial cross limbs project centrifugally the whole length, whilst the four interradial 

 oral columns (ac) project inwards centripetally. The largest horizontal diameter of the 

 gastral hollow space (in the perradia) amounts, both at the base of the stomach and at 

 the oral opening, to from 22-34 mm., whilst the smallest diameter (in the interradia) 

 amounts to only half as much, 11-12 mm. The entire height or length of the oeso- 

 phagus is still less, amounting from the base to oral margin, only from 8-10 mm. 



The gelatinous fulcral plate (zw) is strongly thickened in the upper half of the 

 subumbral gastral wall, and forms several depressed elevations (fig. 5, gw) on its inner 

 surface. It is very delicate and thin, however, in the lower oral half. The circular 

 muscular layer of the oesophagus is also only slightly developed. As in Nauphanta 

 (PI. XXVIII. fig. 14), there is a circular constriction in the middle of its length, 

 which divides the oesophagus into two chambers, shaped like truncated pyramids, which 

 are connected by their narrow bases. We may, perhaps, compare this circular stricture, as 

 in Nauphanta, with the palatine door of the Tesseronise, in which case the lower chamber 

 (which widens below, towards the mouth) must be regarded as the buccal stomach or oeso- 

 phagus, and the upper chamber (which widens above, towards the bottom of the stomach) 

 as the central stomach fused with the basal stomach. In transverse section, through the 

 circular stricture or palatine opening (fig. 6), the largest (perradial) diameter of its 

 cruciform lumen only measures 15 mm., the smallest (interradial) only 6 mm. Below the 

 circular stricture the thin perradial walls of the buccal stomach project, inflated to the out- 

 side, and form buccal pouches (fig. 3, bb) which are separated by interradial buccal columns 

 projecting inwards (wc), as in Periplujlla (Pis. XVIII.-XX.), but not so strongly developed. 

 Above the palatine opening, the central stomach arches outwards perradially in the 

 same way, corresponding to the characteristic crossform of the gastral covering. 



The covering of the stomach, or that part of the endodermal surface of the central 

 gelatinous umbrella, which forms the upper (aboral) wall of the quadrangularly prismatic 

 oesophagus, shows, when the latter is removed, the distinct crossform shown in fig. 6 

 and the centre of fig. 3. The four perradial limbs of the regular cross are rounded off, 

 almost circularly, and are separated by the four triangular septa, projecting inwards, 

 which on account of their special importance, we shall immediately describe more 

 minutely as "cathammal plates " (It). In the middle, between the cathammal plates, 

 four broad tangential transverse clefts remain at the distal end of the four cross-limbs; 

 these are the four perradial gastral openings (figs. 3, 6, go) through which the central 

 stomach opens into the peripheric coronal intestine. 



