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



columns (" columnse orales," ae) have a tendency to centripetal growth, project inwardly, 

 and end below in the archings of the oral margin. 



§ 111. Oral opening (" actinostoma, apertura oris, osculum," eta). In all Medusae, the 

 mouth is originally a simple, usually quadrate or cruciform opening at the lower end of 

 the buccal stomach. Its margin, however, rarely continues perfectly simple, different 

 organs being commonly developed from it, of which the four perradial oral lobes and the 

 oral arms, which have arisen from their prolongation, are by fax the most usual and the 

 most important (§ 113). The terminal oral opening itself in the Medusae usually shows 

 the same characteristic cross figure as the transverse section of the oesophagus, the 

 typical oral cross ("stomostaurus"), with four limbs projecting perradially and four 

 intermediate angles projecting interradially (PL I. figs. 2, 4 ; PI. XV. figs. 5, 6, &c). The 

 perfectly constant position of this oral cross is very important for the orientation of the 

 transverse axes. The free oral margin or the margins of the oral arms are usually 

 strongly armed with thread cells, which are often placed in special regularly distributed 

 groups (PL I. fig. 4). As the gelatinous supporting plate below these groups of thread 

 cells is arched conically or hemispheroidally, oral urticating papillae or urticating 

 knobs are formed (oral papillae, "papillae orales," e.g., in Pelagia). If these 

 grow in length, they become developed into the tentacle-like, cylindrical movable 

 filaments, which serve, like the true (marginal) tentacles, both for feeling and for seizing 

 upon prey. The structure of these oral tentacles or oral fingers (" digitella ") completely 

 resembles that of the inner "gastral tentacles," or gastral tentacles (§ 107) with which they 

 were formerly often confounded (" oral filaments "). But the epithelium, which covers 

 the solid gelatinous axis of the two analogous organs, belongs to the endoderm in the 

 gastral filaments and to the ectoderm in the oral digitella. In many Semostomas {e.g. 

 Aurelia) and in all Rhizostomae, a large number of digitella beset the margins of the oral 

 arms. The oral styles (" stomostyli ") are apparently similar, but essentially different 

 organisms. They are developed principally in the Anthomedusae, where they characterise 

 the families of the Margelidae and Dendronemidae (System, p. 70, taf. v.-vii.). In 

 structure they completely resemble the solid marginal tentacles, and consist of a 

 cylindrical axis, formed of a single row of endodermal chordal cells (PL I. fig. 5, d) ; 

 these are separated by a firm, elastic supporting plate (z), from a thin muscular plate (m), 

 whose longitudinal fibres are connected with the ectodermal epithelium (q) ; the free distal 

 end bears a spheroidal urticating knob (m). There are originally only four simple oral 

 styles present at the four perradial oral angles (Oytceis, Lizusa, System, taf. v.-vii.). 

 The oesophagus is sometimes prolonged secondarily into a long pendant proboscis, so that 

 the oral styles, which were originally terminal, are found at its base (Thamnostylus, 

 PL I. fig. 1 ; Limnorea, Nemoposis, System, pp. 86, 92, taf. v.). 



§112. Buccal pouches and oral columns ("bursae buccales" et " columnae orales"). 

 In some Medusae of both sections, the thin, extensible walls of the oesophagus are 



