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



tissue. All the different forms of the " cellular connective tissue " in the higher animals 

 (bones, cartilage, vascular tissue, mucous tissue, &c), belong to this filling tissue or 

 malthar tissue ; in the Medusae it actually appears only in two essentially different forms, 

 as gelatinous tissue and as chorda! tissue. The cellular gelatinous tissue ("telagela- 

 tinosa ") is the more important as to extent and distribution. It forms the principal mass 

 of the gelatinous umbrella (and therefore of the whole body) in the majority of the Acras- 

 pedss (namely, in most of the larger forms), whilst it is replaced by the cell-less "fulcral 

 gelatinous tissue " in the majority of the Craspedotae. The cells of the " malthar gela- 

 tinous tissue " are usually scattered sparsely at great distances in the structureless inter- 

 cellular substance, but sometimes also in greater numbers (namely, near the cathamma) 

 (PI. XXV. fig. 10). They usually proceed from the endoderm, from whose epithelial 

 layer they have passed into the underlying fulcral layer (" endodermal secreted layer," 

 principally in the umbrella and subumbrella). Similar " ectodermal secreted tissue, 

 " whose cells proceed from the ectoderm (as in the velum of the Pectyllidae, PI. V. fig. 

 7, x ; PI. VI. fig. 13, as), are more rarely found. The consistency of the gelatinous tissue 

 varies greatly, as on the one hand it may become extremely soft mucous tissue (e.g., the 

 umbrella of the Aurelia), and on the other, a very firm, hard fibrous cartilage (e.g., the 

 cathamma of the Peromedusae (PI. XXV. figs. 8, 10). Near these firm fused ridges, the 

 cathammal plates in particular, the gelatinous tissue of many Acraspedae acquires a nature 

 which so resembles the true " fibrous cartilage " of the vertebrates both in histological 

 structure and physical quality as to be easily confounded with it. In this case the extra-* 

 ordinary firmness of the cellular tissue is chiefly formed by thickening and by the fibrous 

 differentiation of the intercellular substance, whilst the softer or firmer nature of the gela- 

 tinous tissue seems usually dependent upon the qualitative and quantitative development 

 of the elastic fibres in it. The latter comport themselves in the cellular gelatinous tissue 

 in the same way as in the cell-less tissue and usually pass from the ectodermal on to the 

 endodermal surface of the gelatinous umbrella (PL IX. figs. 5-7, itf). They are either 

 simple or branched, usually cylindrical, more rarely flattened like a ribbon (PI. VI. fig. 

 19). They are sometimes combined into an elastic network or grouped in branches (PI. V. 

 fig. 8, us). The second principal form of the padding tissue is the characteristic chorda! 

 tissue (" tela chordalis "), which greatly resembles the tissue of the " chorda dorsalis " of 

 vertebrates. It is found everywhere in the solid tentacles of the Medusae, and forms 

 their characteristic firm axis. This is usually cylindrical, and consists of a single row of 

 large, discoid, flat, circular endodermal cells, lying one above the other like the coins in 

 a rouleau of sovereigns (PI. I. fig. 57 ; PL VI. fig. 17; PL XII. fig. 11 ; PL XIII. figs. 

 5,6). Each cell is surrounded by a very thick, firm, elastic membrane, and encloses con- 

 tents as clear as water. The protoplasma of the cell is usually limited to a thin wall- 

 layer, lining the inside of the capsule-shaped membrane, and to a central axial cord, which 

 connects the middle of the proximal and distal wall-layer; the two are sometimes connected 



