REPORT ON THE DEEP-SEA MEDUSAE. XXXTli 



III NEUEODERMAL SYSTEM OF THE MEDUSAE. 



§ 57. Composition of the neurodermal system. Of the large organic systems compos- 

 ing the body of the Medusae, the neurodermal system includes the aggregrate animal 

 organs, the apparatus of sensation and motion. This is therefore opposed physiologically 

 to the gastrovascular system, which forms the complex of the vegetative organs. This 

 antithesis is also shown histologically in reference to the two primary germinal layers, as 

 the greater and most important parts of the neurodermal system originate from the ecto- 

 derm (or "animal germinal layer"), whilst those of the gastrovascular system originate 

 chiefly from the endoderm (or "vegetative germinal layer"). The apparatus of motion, 

 formed by the umbrella and the wide-spread muscular plates, situated on the concave 

 surface of the umbrella cavity, is by far the more considerable although the less differen- 

 tiated of the two apparatuses which compose the neurodermal system. The apparatus of 

 sensation is less extensive, but more strongly differentiated ; it is situated chiefly on the 

 umbrella margin, and includes the nervous system along with the tentacles and differenti- 

 ated organs of sense. 



§ 58. Umbrella (u). The typical and most characteristic principal organ of the Medusae, 

 which distinguishes them from the ancestrally-allied polyps, is their peculiar swimming 

 organ, the umbrella. From its volume and weight this always forms the principal mass 

 of the body, and consists of a voluminous gelatinous body (" collosoma ") which contains a 

 large amount of water, and is sometimes almost as soft as mucus, sometimes almost as 

 hard as cartilage. It is more or less " umbrella-shaped," convex above, arched concavely 

 below. The general form of the umbrella, however, varies greatly. Sometimes its vertical 

 diameter (or "the central principal axis") is greater than the greatest horizontal diameter 

 (or the transverse axis), and the umbrella is thin, conical, bell-shaped, pyramidal, or 

 obelisk-shaped (as in most Anthomedusse, Stauromedusae, Peromedusae, and Cubomedusae, 

 Pis. XV.-XXVL). Sometimes, on the contrary, the horizontal diameter is greater than the 

 vertical, and the umbrella, therefore, more discoid, hourglass-shaped, or semi-spheroidal 

 (as in most Leptomedusae, Trachomedusae, Narcomedusse and Discomedusaa (Pis. I.-XIV., 

 XXVII. -XXXIL). The gelatinous body is usually thickest in the middle of the umbrella, 

 and decreases sometimes regularly, sometimes more suddenly towards the umbrella 

 margin. If we take the umbrella and the parts of the gastrovascular system enclosed in it 

 as a whole, we may term the outer convex surface the "outer umbrella" or "exumbrella" 

 (<>), and the inner concave surface the "inner umbrella" or "subumbrella" (w). More 

 accurately speaking, however, the umbrella consists of two distinct gelatinous plates which 

 may be distinguished as the dorsal umbrella ("umbrella dorsalis," or " notumbrella") and 

 the ventral umbrella ("umbrella ventrabs," or "coelumbrella"); the former corresponds 

 to the "calyx" or dorsal wall of the polyps, the latter to their " peristomium " or ventral 

 wall. Both walls pass immediately one into the other only at the umbrella margin, and 



