20 ON THE ANATOMY OF THE FAMILY OF THE MEDUSA 



with their blind ends towards the inner surface of the outer membrane ; 

 internally, they open each by a distinct aperture on the free surface of 

 the inner membrane. 



45. The contents of the sacs are spermatozoa, and cells in every 

 stage of development towards spermatozoa. These stages are — i. 

 Spherical cells, x^jVoth of an inch in diameter, filled with smaller 

 nucleated cells (fig. 23 a). 2. Cells exactly resembling these included 

 cells but free, and about j-oVoth of an inch in diameter {b). 3. Similar 

 cells, occasionally united into ma.sses with long filiform productions 

 {c). 4. Similar cells with a short process in the opposite direction 

 also ; these swim about freely and sometimes move their tails {d). 5. 

 Perfect spermatozoa with elongated heads (yoVoth of an inch), rather 

 larger below than above, where they are not more than yu^u-oth of an 

 inch in diameter, with very long tails of immeasurable fineness, 

 extending from the larger extremity {e). From the existence of these 

 different stages, I conclude that the spermatozoa are formed by 

 the elongation of the secondary cells contained in the large cells first 

 mentioned. 



46. I have not been fortunate enough to meet with any description 

 of the generative organs of the Rhizostomidae except that of these 

 organs in Cephea by Will ; and as what I have observed differs some- 

 what from his statements, I will describe those of Rhisostoma mosaica 

 somewhat fully. 



In this Acalephe, the eight arms which bear the stomachs are 

 inserted into the lower angles of a thick square plate, which I have 

 thence called the " brachiferous plate," fig, 27. From the upper angles 

 of this plate there arise four pillars, of the same structure as the 

 peduncles of the arms, and are inserted into the under surface of the 

 disc rather external to the middle point between its centre and margin. 

 The " brachiferous plate " has no other attachment to the disc, so that 

 it forms the floor of an arched cavity, with four entrances between the 

 suspending pillars of the plate. 



The suspending pillars expand at their attachment to the disc into 

 three thickened ribs or crura, two of which are lateral and external, 

 and one central and internal : these are united by a thin membrane. 

 The central crura meet and form a cross under the centre of the disc ; 

 the lateral crura are continuous with the substance of the disc above, 

 and each meets with its fellow external to the centre of the disc, fie 26. 

 The central crura are united with these and thence with the disc 

 by the thin membrane only. It thence follows that there exists above 

 the central crura and the connecting membrane a wide crucial cavity ; 

 into this the canals of the suspending pillars open, and from it radiate 



