REPORT ON THE SIPHONOPHOR^. 319 



this latter is branched and bears numerous clustered gonodendra, each of which is again 

 composed of a gonopalpon, a large female gonophore, and a variable number of smaller 

 male gonophores. The Nectophysidse, on the other hand (PL XXIIL), have loose cormidia 

 (similar to those of the Halistemmidas) ; the gonostyles are not attached to the trunk 

 at the basal insertion of the siphons, but separated from them on the internodes ; some- 

 times a single gonodendron in the middle between each two siphons [Nectophysa, &c), 

 at other times two to four or more gonodendra. 



Pneumatophore (PL XXIIL figs. 1-4, p ; PL XXIV. figs. 1-7, p).— The float filled 

 with air is in all Rhizophysidse relatively large, much larger than in the Physonectae ; 

 its form is variable, usually ovate, ellipsoidal or pyriform. Its central axis is sometimes 

 vertical, usually more or less incbned, at other times almost horizontal. The pneumato- 

 codon, or the outer wall of the float (p), is separated from the pneumatosaccus, or the 

 inner wall, by a wide cavity, which often occupies nearly the half of the pneumato- 

 phore, or even more. This pericystic cavity is closed above (where the two walls of the 

 float are connected on its top), and opens below into the axial canal of the trunk ; its 

 greater part is filled up by clusters of large branched villi, the radial apophyses of the 

 pneumadenia. 



Pneumatosaccus. — The invaginated part of the exoderm, which we call pneuma- 

 tosac, hangs freely in the pericystic cavity, and is not connected with it by 

 radial septa (as in most Physonectae). It is connected with the pneumatocodon 

 only at its top, where it opens by the apical stigma (po). The simple wall of the 

 young pneumatosac (comparable to the invaginated entoderm of a gastrula) is divided 

 in the adult Rhizophysidas into five different parts, viz.: — (l) A mitra ocellaris or an 

 apical pigment-cap ; (2) the pericystic sac, enclosing the air-flask ; (3) the hypocystic 

 funnel ; (4) the hypocystic radial bunches of villi arising from the funnel ; and (5) the 

 endocystic tapetum (" secondary exoderm "). This latter lines the greater part of the 

 inside of the chitinous pneumatocyst. 



Pneumatocyst. — The chitinous air-flask or pneumatocyst is originally the cuticle 

 of the young pneumatosac. It is ovate, spindle-shaped or cylindrical, and opens at 

 both poles of its longitudinal axis ; the inferior opening is the pylorus infundibuli, 

 which leads into the funnel-cavity of the pneumadenia ; the superior opening is the 

 apical stigma, through which the air may be emitted at will from the cavity of the 

 pneumatocyst (PL XXIIL fig. 3, x). This stigma, or the apical opening of the float 

 (PL XXIV. figs. 4, 5,po), may be opened by a corona of radial muscles (pn) and closed 

 by a sphincter composed of ring-muscles {pm). 



Mitra ocellaris. — The pigment-cap of the float, or the mitra ocellaris (PL XXIIL 

 fig. 3,2JP', PL XXIV. figs. 4, 5,pp), occupies sometimes nearly the upper half, at other 

 times only the uppermost third or fourth of the pneumatosac, and is composed of elegant 

 polygonal pigment-cells, separated by colourless intervals. The colour is usually red or 



