REPORT ON THE DEEP-SEA MEDUSAE. 123 



nearly closed pouch, whilst the inverted ventral fold of the fulcral shield extends axial- 

 wards as far as the root of the sterigma with which it is fused in great part. At one 

 single small spot only there remains a narrow opening, which leads from the hollow 

 space of the coronal sinus (cs) into the genital sinus (ss). This narrow opening, 

 the " apertura sinus genitalis " (fig. 3, left, so) lies at the point where the sterigma root 

 (st) touches the proximal margin of insertion of the interradial deltoid muscle (rad"). 

 The two sinus apertures of the two connected genitalia (of a pair) therefore He near each 

 other, and are only separated by the insertion of the interradial deltoid muscle (md"). 

 The ova are developed from the germinal epithelium in the hollow of the sinus on the 

 concave side of the shield and essentially in the same way as in Nauphanta and the 

 other Cannostomse. The Mediterranean Nausithoe, which has never been sufficiently 

 minutely investigated either in this respect or many others, shows no essential 

 difference on this point. 



The germinal epithelium of the endoderm, from which the ova in Atolla are 

 developed, lies exclusively outside the genital sinus, on the concave inner side of the 

 pouch-shaped folded sterigma, whose convex outer side is only covered by the usual 

 sterile endoderm of the subumbral wall of the coronal sinus. The youngest ova lie 

 inside the genital sinus, on its proximal margin, the oldest ova on the distal margin, 

 both on its dorsal and its ventral wall. We consequently find a double fertile 

 germinal zone, which lies tangentially near the root of the sterigma, and forms a 

 transverse ridge standing perpendicularly on the adradius. The youngest tangential 

 rows of ova, which are followed to the outside distal walls (both on the dorsal and 

 the ventral wall of the fulcral shield) by rows of older ova, lie on both sides of this 

 neutral germinal zone. The oldest and ripest ova lie on the radial margin (radial 

 section, fig. 9, so). The younger ova (on the proximal margin) are completely embedded 

 in the gelatinous plate of the sterigma, and therefore lie in closed fulcral capsules, as 

 in Periphema (PL XXV. fig. 7). This is equally the case in the closely allied 

 Nausithoe and Nauphanta (PI. XXVIII. fig. 15). The ripe ova of Atolla are very 

 large and spheroidal, they are more than a millimetre in diameter, and contain a visible 

 food-yolk. When completely ripe they fall into the genital sinus (ss), from which 

 they are emptied through the narrow aperture (sa) into the coronal sinus (cs) and 

 thence pass outside through the stomach and mouth. The ovaries both of Atolla and 

 of Nauphanta were, unfortunately, badly preserved, the epithelium being almost 

 entirely destroyed, so that the finer structure could not be satisfactorily investigated. 

 The peculiar finer structure and development of these very old Cannostomas appears, 

 however, to present an essential homology with those of the Peromedusae. In both the 

 former (Ephyridae and Linergidse) and the latter (Pericolpidaa and Periphyllidaa) the 

 remarkable and complicated conditions of the genitalia merit more minute investiga- 

 tion of well-preserved material. 



