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



layers of the cathammal plate remain distinct during the life of the Medusae, and can 

 even be artificially separated (by suitable pressure) in fine transverse section. This is 

 however not usually the case. Only a single thin layer of cells is generally visible in 

 the connective tissue of the cathamma, as the two endoderni plates, which were originally 

 separate, have become completely fused together. A considerable hardening and 

 thickening of the two connective plates usually takes place in the cathamma on both 

 sides of the endoderm plates, and the soft gelatinous tissue is sometimes even trans- 

 formed into true firm fibrous cartilage (PI. XXV. figs. 8, 10, wg 2 , zw 2 ). 



§ 102. The three principal forms of the cathamma (k). The fusion of the two walls of 

 the gastral space, which give rise to the cathamma or septa, may appear in three 

 principal forms, according as they take place in a point, a line, or a surface. In all three 

 cases the original number of the cathamma amounts to four, and these four primary septa 

 lie interradially (in the middle between the four primary tentacles), whilst the four 

 primary radial cavities separated by them lie perradially (in the same meridian planes as 

 the four primary tentacles). The number of the cathamma may, however, be considerably 

 increased secondarily (corresponding to variations of the homotypical fundamental numbers 

 already mentioned, §§ 23-26). In the most simple case when the concrescence takes place 

 in four points, four septal walls are found (" nodi cathammales," hi) as in part of the 

 StauromedusEe and in all Peromedusse. The peripheric hollow space of the gastro- 

 vascular system then appears as a large coronal sinus (" sinus coronaris "), whose division 

 into four is only indicated by the four small nodes (Pis. XV., XX., XXV.). In the second 

 case, when the fusion takes place in four lines, four septal ridges (" limites cathammales," 

 hw) are found as in most Stauroniedusse, all Cubomedusse, and part of the Disconiedusse 

 (Cannostomas, and half of the Semostomse, Pelagidse, and Cyaneidse) ; the peripheric 

 hollow space of the gastrovascular system then forms four radial pouches (" bursas 

 radiales "), which are separated by the narrow ridges (Pis., XVI., XVIL, XXVI.). In the 

 third case, when the fusion takes place in four surfaces, four septal plates (" tabulae 

 cathammales," kt) are formed ; as most Craspedotse, and among the Acraspedae, in part of 

 the Discomedusa? (in half of the Semostomaa : in the Flosculidse, and Ulmaridae, and in 

 all the Rhizostomse) (Pis. I., II., XXXIL). 



§ 103. Cathammal fissures (" antra septalia "). In the order of the Anthomedusse 

 (and in this only) there appears frequently, if not universally, a partial dissolution of the 

 cathamma, and consequently a local separation of the two fused umbrella walls, 

 by which the peculiar cathammal fissures or septal cavities of these Craspedotse are 

 originated. In all the cases hitherto observed, they appear as eight adradial cavities, 

 which are completely closed, and contain a gelatinous fluid. They occupy the greater 

 part of the subumbrella, are limited below by the umbrella margin, above by the base of 

 the oesophagus, and are separated from one another by the eight band-shaped longitudinal 

 muscles of the subumbrella, of which four run perradially on the axial side of the four 



