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



where, which leads, as in higher animals, to the formation of different tissues, and these 

 tissues are regularly arranged in four strata, which in a certain sense may be considered 

 as " secondary germinal layers." 



§ 41. Secondary germinal layers. If we consider the histological differentiation and 

 the structure of layers connected with it, of the organism of the Medusae as a whole, and 

 if we disregard the close connection of the different " principal layers " and " secondary 

 layers," we can universally distinguish accurately four layers, which follow one another 

 from the exterior, inwards : — (1) the dermal plate or dermal covering (" lamina chrotalis," 

 " chrotoderma"), the layer of cells which covers the whole outer surface of the body in 

 continuous connection and passes into the endoderm at the oral margin ; (2) the muscular 

 plate (" lamina muscularis," " myoderma "), the thinner or thicker muscular layer, which 

 proceeds from the ectoderm and is chiefly spread over the concave lower side of the 

 umbrella ; (3) the connective plate (" lamina connectiva," " colloderma "), the gelatinous 

 or cartilaginous mass secreted from the endoderm, which as a thicker " gelatinous 

 umbrella " forms, according to volume and weight, the principal mass of the body in all 

 Medusae, but which is also found as the thinner " supporting lamella " in the subumbrella 

 and the tentacles ; (4) the intestinal plate or intestinal epithelium (" lamina gastralis," 

 " gastroderma "), which lines the whole inner upper surface of the umbrella in continuous 

 connection, and passes into the ectoderm at the oral margin. Although the muscular 

 plates are for the most part produced from the ectoderm, and the connective plates, on 

 the other hand, from the endoderm, we must distinctly remember that in isolated cases it 

 is reversed, and muscles are formed from the inner germinal layer and supporting plates 

 from the outer. 



§ 42. Two opposite views may be held in histologically judging the organism 

 of the Medusae. On the one hand, the Medusae may be considered as diblastic animals, 

 as, in the majority of them, all the tissues which appear between the two primary ger- 

 minal layers remain in close connection with them, are referable with certainty to one of 

 the two, and only acquire a slight amount of independence. But, on the other hand, we 

 may consider part of the Medusae (and in a certain sense all of them) as mesodermal (tri- 

 blastic or tetroblastic animals), as in certain parts of the body (and in some Medusae to a 

 great extent) independent tissues are really secreted between the outer and the inner ger- 

 minal layer, and so form a middle germinal layer (" mesoderma "). The following tissues 

 may be pre-eminently regarded as independent mesodermal tissues : — (l) the gelatinous 

 tissue of the umbrella as soon as it contains independent cells ; (2) the chordal tissue in 

 the axis of the solid tentacles ; (3) the muscular tissue of isolated, especially strongly 

 developed muscles ; (4) the nervous tissue in a part of the nerve centres and the organs 

 of sense. In a secondary degree, but less accurately, the following may claim to be 

 termed mesodermal tissues : — (1) the reproductive tissue ; (2) part of the urticating 

 tissue (the subepithelial urticating organs) ; (3) all subepithelial muscles ; (4) all 



