REPORT ON THE DEEP-SEA MEDUSAE. 



THE ORGANISATION OF THE MEDUSA. 



Sketch of a Comparative Morphology of the Medusae as an Introduction to the 

 Description of the Deep-sea Medusa of the Challenger Expedition. 



I. GENERAL MORPHOLOGY OF THE MEDUSiE. 



§ 1. Definition of the Medusas. Medusas are Cnidarias or Acalephae with a gelatinous, 

 radially constructed, concave-convex umbrella, whose vertical axis is the principal axis 

 of the single persona ; with swimming muscles on the concave oral side of the umbrella ; 

 nerve centres and organs of sense on the peripheric margin of the umbrella ; with radial 

 processes (canals or pouches) of the central gastral cavity, and a simple (seldom multiple) 

 oral opening at the oral pole of the principal axis ; also with genitalia in the subumbral 

 wall of the gastrovascular system (comp. System der Medusen, 1879, p. 21, &c). Medusae 

 are distinguished from the other classes of Acalephae or Cnidarise (Polyps, Corals, Siphono- 

 phora, Ctenophora) by the following class characters : the mature, completely developed 

 Medusa appears as a single persona (not united in numbers in a colony), which usually 

 swims freely in the water, seldom crawls, still more rarely is fastened to the bottom ; 

 (nearly all Medusas live in the sea, only a few in fresh water). The principal mass of the 

 Medusa body (by volume and weight) forms a concave-convex, watery gelatinous body, 

 the umbrella. This serves as the special swimming organ ; a vertical principal axis and 

 two or more horizontal transverse axes (with twice as many radial axes) are always the 

 standard for its pyramidal base form. The convex outer surface of the umbrella (exum- 

 brella) does not generally bear special organs, whilst the concave inner surface (sub- 

 umbrella) is always overlaid with a muscular plate, which, as the most important organ of 

 motion, deepens and narrows the umbrella cavity by its contraction, and thereby expels the 

 water from it. The most important organs of sensation, the nerve centres, and differenti ■ 

 ated organs of sense, and generally the tentacles also, lie on the umbrella margin (where 

 the exumbrella and the subumbrella are contiguous). The organs of nutrition are 

 formed by a radial gastrovascular system which extends over the subumbrella, and is 

 composed of a central principal intestine and a peripheric coronal intestine. The principal 

 intestine (in the centre of the subumbrella) forms a simple gastral cavity, which often pro- 

 jects below like a tube and opens at the oral pole of the principal axis by a simple (seldom 

 multiple) mouth. The coronal intestine (in the periphery of the subumbrella) is composed 

 of four or more radial processes of the central stomach, which sometimes take the form of 

 broad pouches, sometimes of narrow canals. The organs of reproduction are always simple 

 reproductive glands (genitalia) and are always developed in the lower (subumbral) 

 wall of the gastrovascular system, sometimes from its endoderm, sometimes from its 



