202 ROBERT PAYNE BIGELOW ON 



ANATOMY. 



Form of the Body. To one who is familiar with the cyaneas, aurelias, and the 

 like, of our northern coast, the shape of this medusa appears very strange. The aboral, 

 or exumbrellar surface (Fig. 35), instead of being convex in Gassiopea xamachana, as 

 it is in the great majority of medusae, is concave when the animal is at rest, except for a 

 slight convexity over the stomach, and except in the region of the thinner marginal part 

 of the umbrella, where also it is convex. The surface of the subumbrella, on the other 

 hand, is convex, except in this same thinner marginal area, where it is in turn concave. 

 The umbrella thickens very gradually from its margin to the centre, and the elevations 

 and depressions of its surface have very gentle slopes, so that its general shape is much 

 nearer that of a flat disc than the dome-like form of most medusae (Fig. 35). 



A circular column arises from the centre of the oral surface of the umbrella. It is 

 broad, but very short; and a few millimeters from the umbrella it loses its circular 

 outline, owing to eight stout arms that spring from it at regular intervals (Fig. 34). 

 These are smooth and rounded, except along a line on the oral side, where they bear 

 the fringe of oral appendages, and they are long and much branched. This column is 

 the oral disc, and its arms the oral arms. 



TJie Structure of the Mesogloea. By far the greater part of the mass of the oral 

 arms and disc, as well as the umbrella, is composed of a firm, elastic, gelatinous 

 substance, the mesogloea, and it is to this that the shape of the body is due. 



The description given by Keller ('83), of the structure of the mesogloea in G.poly- 

 poides would apply almost equally well to our species. The mesogloea consists of a 

 hyaline matrix, in which are imbedded certain fibres and three kinds of cellular elements. 

 Most of the fibres appear to be analogous to connective tissue fibres, and take a general 

 course through the mesogloea at right angles to the surface. Others seem to be proto- 

 plasmic. At any rate, they may be observed to proceed from the star-shaped cells that 

 are scattered throughout the jelly. 



The cellular elements are: the star-shaped cells, just mentioned; vesicular bodies, 

 found in certain restricted localities ; and the green cells, which, as it will be shown later, 

 are symbiotic plants. 



The star-shaped cells remind one of osteoblasts, and are probably analogous to them, 

 in that they are concerned in the formation of the jelly. Hamann ('8l) has called 

 them colloblasts. They are small, often somewhat elongated, and have a well-marked 

 nucleus. 



