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



they lie on its inner endodemial subumbral wall (fig. 9, iv) ; each genitalium is connected 

 continuously with the wall, only at a single, limited spot, which we shall call the root of 

 the genitalia (" sterigma," fig. 3, st; fig. 9, st, in radial section) as in Nauphanta. 



Finer anatomy, moreover, shows an extremely complicated structure of the ovarian 

 pouches, instead of the apparently simple conditions. The very thin subumbral wall ot 

 the coronal sinus (cs) only stretches superficially, like a debcate folded veil, freely over 

 the lower surface of the genitalia (fig. 3, right half). The structure of the ovaries is most 

 nearly allied to that of Nausithoe and Nauphanta, but is also closely connected with 

 that of the Peromedusse (Periphylla and Periphema) ; it shows, moreover, peculiar 

 complications, which are difficult to understand, and do not occur in other Discomedusse. 

 We can distinguish two principal component parts in each ovary, the gelatinous fulcral 

 frame or sterigma, and the endodemial germinal epithelium, which covers the sterigma, 

 and produces the ova ; the former is a process of the gelatinous supporting plate of the 

 subumbrella of the coronal sinus, the latter is a local production of the endodermal 

 epithelium of the supporting plate. 



The " sterigma " (st) or the cartilage-like fulcral frame of the ovary gives the latter 

 its characteristic form and corresponds to the " costa genitalis" of the Perornedusae, 

 to the "sexual axial plate " of the Cuboniedusas, and to the "genital fold" of many 

 Discomedusae. It consists of a strong scutiform gelatinous plate, hard as cartilage, and 

 shaped like a kidney It is from 10-12 mm. long (in tangential direction), 6-7 mm. broad 

 (in radial direction), and 1-2 mm. thick (in vertical direction). Like the scutiform 

 sterigma of Nauphanta (Pis. XXVII. , XXVIII. figs. 4-15), the sterigma of Atolla hangs 

 freely, for the most part, in the hollow space of the coronal intestine (and here, 

 therefore, of the coronal sinus), and is only connected with its subumbral wall at a 

 single point. This spot, the root of the sterigma (st), is a narrow ridge, pigmented 

 brown, from 5-6 mm. long and ^— | mm. broad (fig. 3 above to the left, st). It 

 nearly fills the interspace between the proximal end of a perradial deltoid muscle 

 and that of an interradial deltoid muscle, and forms part of the distal base-line of a 

 distal cathammal plate (let), whilst the middle third of this base-line serves as the 

 base of insertion of an interradial deltoid muscle (md"), and therefore presents a com- 

 plete homology with the septal nodes of the Perornedusae, whilst its two lateral thirds 

 are occupied by the sterigma roots of two ovaries which belong together. Like the 

 whole equilaterally-triangular cathammal plate (kt), its base consists of a concrescence 

 of the umbral and subumbral wall of the umbrella, immediately below which the car- 

 tilage-like gelatinous ridge, which forms the root of the sterigma, rises and then extends 

 scutiformly. In Atolla as in Nauphanta, this fulcral shield projects, strongly arched 

 into the hollow space of the coronal sinus and turns over at its convex distal margin 

 (where it nearly touches the corona of pouches) in such a way as to originate a nearly 

 closed genital sinus (fig. 9, ss). This sinus, therefore, assumes the form of a fiat, 



