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



of the latter is often as strongly thickened as that of the former, and frequently becomes 

 as hard as cartilage, especially in the larger Acraspedee. In many Cyaneidse it forms 

 a thick palatine ring as hard as cartilage (" annulus palatums," PI. XXX. figs. 30, 31, aw). 

 The Cubomedusas Chirodropus is distinguished by the development of its palatine lips, 

 into four intcrradial palatine valves (" valvulse palatinse," System, p. 429, taf. xxvi. 

 figs. 3, 4, k) ; they resemble the semilunar valves of the human heart in form, have their 

 concave side turned to the central stomach, which they can close completely and so 

 prevent any communication with the oesophagus. 



§116. Central stomach or central cavity (" gaster centralis, cavitas centralis, gc). 

 The second and middle chamber of the three chambers of the axial principal stomach, the 

 central stomach, is homologous in the free-swimming Medusa, with the central calyx 

 cavity, or the true stomach of the sessile polyps. It is separated from the buccal 

 stomach above by the palatine opening (" palatum "), from the basal stomach, below by 

 the pyloric opening ("pylorus"). In the side walls of the central stomach there are 

 four perradial (rarely more) openings ("ostia gastralia") by which it communicates with 

 the surrounding pouches or canals of the coronal intestine. The general form and 

 relative size of the central stomach varies most remarkably, and is often difficult to define. 

 It usually forms the widest and broadest of the three chambers of the principal intestine, 

 whilst the buccal stomach is the longest. The central stomach is sometimes entirely en- 

 closed in the gelatinous substance of the umbrella, sometimes not ; in the former instance 

 its horizontal axis is usually considerably larger than its vertical, in the latter instance it 

 is usually the reverse. We can therefore generally distinguish two principal forms of 

 the central stomach, which are, however, connected by numerous intermediate forms and 

 cannot be sharply defined, — the high obelisk stomach, and the flat lens stomach. The 

 high obelisk stomach (" gaster centralis obeliscus ") has usually the form of an obelisk or 

 a truncated quadrate pyramid (Pis. XV.-XXIV.) ; the palatum forms its lower base, the 

 pylorus its upper base, the four perradial gastral ostia correspond to the four angles of 

 the obehsk, the four interradial side walls of the central stomach or the obelisk plates 

 (" tabula? obelisci," gz) to the four sides. The vertical axis of the obelisk stomach is 

 usually larger than the horizontal diameter. It usually hangs freely in the umbrella 

 cavity and is often fastened to a shorter or longer " gastral peduncle " (" pedunculus 

 gastralis," «s, see above) ; this is the case in the majority of the Anthomedusas (System, 

 taf. iii., iv. ; vii.), of the Leptomedusae (System, taf. xi.-xv.), and of the Trachomedusse 

 (System, taf. xvi.-xviii.), also among the Acraspeda? in most Tesseronise, in the 

 Stauromedusse as well as the Peromedusse and Cubomedusae (System, taf. xxi.-xxvi.). In 

 many cases four perradial mesenteric folds or mesogonia serve to fasten the freely hanging 

 obelisk stomach to the subumbrella ; the Pectyllidae are distinguished by eight such 

 mesogonia (four perradial and four interradial) (Pis. III.-VTIL). The second principal 

 form of the central stomach, the flat lens stomach (" gaster centrabs lenticula ") only 



