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



the tubular siphon is always very extensible and contractile, provided with strong longi- 

 tudinal and circular muscles. The entoderm of the stomach usually bears very numerous 

 and small hepatic villi, containing pigment-granules and clear glandular vacuoles ; rarely 

 the hepatic glands are arranged into eight or sixteen longitudinal series, and form 

 coloured " liver-ridges," as in Linophysa. The distal mouth is always very dilatable, and 

 may be expanded in the form of a large circular suctorial disc (PL XXIII. fig. 5 ; 

 PL XXIV. fig. 1). 



Tentacles. — The long tubular tentacle which arises from the base of each siphon, on 

 its superior or dorsal side, is rarely a simple, unbranched filament, similar to that of 

 Apolemia, as in Linophysa (40, Taf . i. fig. 1 ). In all other known genera it is beset with 

 a series of numerous equidistant tentilla or unilateral branches. These tentilla exhibit 

 in the various genera of Rhizophysidse similar differences in form and structure to those 

 seen in the Agalruidse among the Physonectse, although a true cnidosac (as in the latter) is 

 not developed. The tentilla are simple tubular filaments, with a unilateral series of cnido- 

 cysts, in Aurophysa and Nectophysa (PL XXIII. figs. 1-6); usually the axial or ventral side 

 of the tentillum is beset with several rows of sensitive palpoblasts (fig. 6, tw), the abaxial 

 or dorsal side with some series of spherical cnidoblasts (fig. 6, he). The genera Canno- 

 physa and Pneumophysa are distinguished by trifid tentilla (PL XXIV. figs. 8, 9) ; 

 their distal end bears an odd terminal vesicle (fig. 9, to) and two paired lateral 

 horns (fig. 9, tc) ; these are armed in different ways with cnidonodes or roundish 

 groups of spherical cnidocysts. Rhizophysa, finally, is distinguished by the compound 

 structure of the tentacles which bear two, three, or more different kinds of tentilla ; 

 these are partly simple, partly trifid or branched; and usually there are scattered between 

 them a small number of very large and remarkable appendages, first described in 

 the Mediterranean Rhizophysa jiliformis by Gegenbaur (7, Taf. xviii. figs. 7-9). They 

 are flat, palmate, or hand-shaped leaves, dichotomously branched at the free distal 

 margin, each branch provided with a spherical cnidocyst. In the similar Atlantic Rhizo- 

 physa planostoma (Peron), I observed in December 1866 similar but larger appendages, 

 which bore in the middle of their upper or outer side a large purple ocellus ; a pigment- 

 ring with a strongly refracting globule in the centre. I suppose that this globule is a 

 lens, and the ocellus a true eye. 



Gonostyles. — Each cormidium of the Rhizophysidse bears usually a single, clustered 

 and monoclinic gonodendron only; this is attached to the node of the stem, immediately 

 beyond the basal insertion of the siphon, in the ordinate cormidia of the Cannophysidse 

 (PL XXIV.) ; whilst it is attached to the internodes of the stem, between the siphons, in 

 the loose cormidia of the Linophysidse, where usually the siphons and gonodendra 

 alternate regularly, in equal numbers (PL XXIII. figs. 1-3); but sometimes the number 

 of gonodendra is augmented (in Rhizophysa), so that a variable number of gonostyles (two 

 to four or more) arises from the internode between every two siphons; in some species 



