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



sixteen subradial marginal lobes, inserted between the former and the latter. These organs 

 in all Peromedusee, show very peculiar and complicated conditions of structure, which 

 thoroughly distinguish them from both the Stauromedusse and the Cubomedusse. Notwith- 

 standing, these structures are phylogenetically derived from those of the Stauromedusse 

 and then from those of the Pericolpidse (comp. my System, 1879, taf. xxiii.). The 

 four perradial tentacles and their four interradial sense clubs have arisen from the eight 

 principal tentacles of the Tesseridaa, and are therefore homologous with the "marginal 

 anchors" of the Lucernaridse ; on the other hand the eight adradial tentacles with the 

 alternating marginal lobes of the Pericolpidse, are homologous with the hollow arms of the 

 Lucernariche. The umbrella margin of the Periphyllidse has plainly arisen in this way 

 from that of the Pericolpidse, as instead of each perradial tentacle, there are three 

 tentacles with two additional marginal lobes between them. The number of tentacles 

 rises in this way from four to twelve, and the number of the marginal lobes from eight 

 to sixteen. The original number of the four sense clubs remains invariable in all Pero- 

 medusae, and is typical of the whole order. 



The sixteen marginal lobes ("lobi marginales," PL XVIII. fig. 1 ; PI. XXII. fig. 22 ; 

 PI. XXIII. figs. 29-32) of the corona of the umbrella are, on the whole, egg-shaped, lie 

 subradially in the meridian plane of the fourth order, and are divided into four pair of 

 ocular lobes (lo), and four pair of alternating tentacular lobes (It). The two ocular lobes 

 (" lobi oculares ") of each umbrella quadrant lie exradially, as the interradial eye is set 

 between them. The marginal incision between them up to the eye is 17 mm. deep, half 

 as deep as the incision between each ocular lobe and the neighbouring tentacular lobe. 

 The two ocular lobes of each pair therefore compose a broadly oval, ocular principal lobe, 

 whose free margin is divided into two secondary lobes (fig. 1). Each ocular secondary 

 lobe is divided by a deep longitudinal furrow into two halves, an adocular and an exocular. 

 The exocular or external half, which touches the adradial tentacle, is wing-shaped and 

 thinned, and runs into a delicate membranous selvage in the margin of the lobe 

 (" patagium," Ip). The inner or adocular half, which touches the interradial eye, is 

 strongly thickened, so that in conjunction with that of the adjacent secondary lobe, it 

 forms a thick swelling, projecting convexly outwards, in direct prolongation of the ocular 

 pedalium (ur). Each ocular principal lobe therefore appears like a broad oval leaf, which 

 has a midrib 1 cm. broad, side-wings 3 cm. bi'oad, and is 4 cm. long in all (in the 

 middle line). The two tentacular lobes ("lobi tentaculares ") of each quadrant of the 

 umbrella lie coradially, as they enclose the perradial tentacle beween them and are 

 divided externally from the ocular lobes by the adradial tentacle. Each of the two 

 tentacle lobes represents a longish oval leaf 4 cm. long and 2 cm. broad, divided by a deep 

 subradial longitudinal furrow into two lateral swellings ; this furrow forms the direct pro- 

 cess of the coradial furrow, which separates the adradial pedalia of the furrow zone from 

 the perradial. Each of the two swellings of each tentacular lobe is almost linear. A very 



