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



Plate XIX. ; it forms a quadrangle with rounded angles. The delicate, thin membranous 

 margin of the mouth (oral margin) is contracted inside like a narrow velum, and appears 

 swollen and thickened at the four interradial angles by the crescentic oral ends of the 

 buccal columns which are concave inside. Each of the latter bears two thin oral filaments 

 two cm. long at the end of the horns of the crescent (" barbulse, filamenta oralia," figs. 9-1 1 , 

 «/). These may probably be considered as the last oral branches of the limbs of the 

 tasniola (see below). They are thickened conically at the base, and run out to the point 

 in a very thin long filament (or in a pencil-shaped bunch of filaments) ; they are amply 

 furnished with large bean-shaped nematocysts, whose urticating threads are twisted 

 spirally and armed with bristles. 



The oral cavity (" cavitas buccalis ") is divided by the four interradial buccal columns 

 into four perradial peripheric buccal pouches (bb), which only communicate with the cen- 

 tral cavity of the mouth (ax) by four narrow oesophageal clefts. The four oral columns 

 ("columnse buccales," ac, figs. 9-11, 19) are nearly rectangular ridges or plates, 5 cm. 

 hio-h and 2-3 cm. broad, projecting inwards in the interradial meridian plane into the oral 

 cavity. They are supported by a visible layer of gelatinous substance, several m il limetres 

 thick, which is thickest at the two lateral margins and in the middle of each plate, so that 

 each plate is also traversed on its gastral surface by a pair of shallow, parallel, longitudinal 

 grooves (transverse section, fig. 19, ac). The lateral parts of the buccal columns project 

 like wings on the two sides of the groove (adradial oral wings, "ake buccales," ad). 

 In this respect they resemble the taeniola of the Scyphostoma, and, in fact, I consider them 

 homologous with the peristome part of the latter. In Periphylla mirabilis, moreover, 

 they are much less strongly developed than in the following species : — Perip>hylla regina 

 (PL XXIV. fig. 3) and Periphylla hyacinthina (System, 1879, taf. xxiv. fig. 14). The 

 four perradial egg-shaped buccal pouches ("bursas buccales," bb, figs. 9-11, 19) project arch- 

 ing out externally between the buccal columns. The central spaces only of each buccal 

 pouch opens freely into the oral cavity, then- peripheric spaces have dilatations or horns 

 which are covered for the most part by projections of the enclosing wall. Each buccal 

 pouch is therefore divided by the projecting wings of the buccal columns into the open 

 central space, and the lateral horns or wing pouches (" ventriculi laterales, bursas alares," bd) 

 covered by the buccal columns. Each side pouch passes above into a larger and deeper 

 aboral corner horn, below into a smaller and shallower oral corner horn ; the former ends 

 caseally in the upper thickened end of the wing of the buccal column (fig. 11, ad). The 

 corner horns are not so depressed in this species as in the following one. The four perradial 

 buccal clefts ("fissuree buccales," ae), by which the four buccal pouches communicate with 

 the central space of the oral cavity, are narrowed in the middle. The buccal pouches are 

 inflated ovally out from them (figs. 9, 10, bb). The perradial wall of the buccal pouches 

 is very much thinned, and is traversed by parallel longitudinal streaks, which are 

 divided by fine transverse streaks into darker cubes (oral glands, fig. 10, ag). This wall 



