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



funnel-cavity opens outside by a pore (or stigma) in the median line of the dorsal side, 

 on the base of the pneumatosac ; the animal can expel the air through this basal 

 stigma voluntarily (compare 33, p. 3, and 84, p. 35). 



Nectophores. — The swimming-bells of the numerous Physonectae exhibit a great 

 variety in number, form, and arrangement. They appear to be wanting in two families, 

 the monogastric Athoridaa (PI. XXI. fig. 5) and the polygastric Anthophysidae (PI. XL); 

 but in reabty they are here replaced by paddling bracts, which are nothing else than 

 modified nectophores. One genus only, Dicymba (PI. XVIII. fig. 1), has two opposite 

 nectophores, similar to Diphyes and Praya among the Calyconectae ; and the same con- 

 dition occurs in the young larvae of Discolabidae (PI. XIX. fig. 8), and probably of many 

 other Physonectae. Usually the number of nectophores is six to twelve or more, in the 

 largest forms thirty to fifty or more. They are arranged in two longitudinal opposite 

 rows in most Apolemidae, all Agalmidae (Pis. XIII. -XVII.), some Nectalidae (Nectalia) 

 and Discolabidae (Physophora). Some other genera of the two latter families have four 

 cruciate longitudinal rows (Sphyrophysa and Discolabe, PI. XIX.). All Forskalidaa 

 (PI. VIII.), and Stephanospira among the Discolabidae, possess a strobiliform nectosome, 

 with several spiral rows of nectophores, arranged like the scales of a fir-cone. The 

 Circalidas (PL XXI. fig. 1), finally, are distinguished by the possession of a horizontal 

 corona of nectophores radially arranged, similar to that of the Stephalidae (PI. VII. 

 fig. 39). The same arrangement, either in a single horizontal corona, or in several flat, 

 densely apposed spiral rings, is seen in the numerous bracts of the Athoridse (PI. XXI. 

 fig. 5) and the Anthophysidae (PI. XI. fig. 1). Sometimes the distal end of these 

 paddling bracts still possesses a rudimentary nectosac {Athoria, Rhodophysa), and thus 

 proves its nature as a modified nectophore. 



Umbrella of the Nectophores. — The fundamental form of the umbrella in all 

 Physonectae is bilateral and composed of two symmetrically equal halves (antimeres) ; 

 but it is at the same time quadriradial, marked by the four radial canals. Usually its 

 principal axis is more or less obliquely inclined towards the axial trunk, sometimes 

 almost horizontally ; so that the position of the proximal apex is higher than that of the 

 distal ostium. The nectophores of most Physonectae are depressed and shortened in the 

 dorso-ventral direction, so that the sagittal axis is shorter than the frontal. Usually 

 there arise from the truncate apex two lateral horns or wings, which embrace the trunk. 

 The ventral (or inferior) face of the umbrella, has a concave groove, and from its middle 

 line, near the apex, arises the short pedicle which attaches it to the stem. More rarely 

 (in the strobiliform nectosome of the Forskalidaj) the pedicle is longer than the 

 nectophore and arises from the very apex. The jelly-substance of the umbrella is usually 

 rather consistent and firm, sometimes almost cartilaginous. 



Nectosac. — The muscular subumbrella of the swimming-bells is of very variable size 

 and form, sometimes little smaller than the umbrella, at other times scarcely half as 



