Chap. II. STRUCTURE OF THE ADULT. 55 



In view to a further discussion of the homologies of these animals, I would call 

 special attention to the fact, that we have here eyes in the peripheric prolongation 

 of the interambulacra, as well as of the ambulacra, and that the angles of the 

 mouth and their ai'm-like appendages extend in the direction of the ambulacral ra}'?. 



The only points in the structure of Aurelia, the correct appreciation of which 

 presents some difficulty, are the relations of the central digestive cavity to the 

 genital pouches and to the oral aperture, and perhaps also those of the ocular 

 apparatus to the system of radiating tubes and to the tentacles. A comparison 

 of the magnified views of young specimens of our Aurelia fiavidula, as represented 

 PI. XP. Fifj. 17 and PI. XP. Fig. 5, with adult specimens, Pis. VI. and VII. Fig. 1, 

 plainly shows, that the central cavity acquires much larger proportions, in comparison 

 to the size of the body as it grows older; for in the adult that cavity occupies 

 about one third of the total diameter, while in the young, it is hardly one sixth. 

 With this change in the relative dimensions, great changes also take place in the 

 outlines and form of the arms which surround the mouth, of the pillars by which 

 they are connected to the lower floor of the body, and of the lower surface of 

 the gelatinous disk forming its upper floor. It has already been stated, that the 

 adit of the main cavity is at first a simple hollow pyramid, with the angles of its 

 opening slightly turned out. These projecting angles soon become pendant append- 

 ages with a lobed margin, and these so-called arms very soon increase so far as to 

 equal in length the semidiameter of the disk, so that, when stretched horizontally, 

 their extreme ends reach to the margin, and when hanging down, they project to a 

 considerable extent below the umbrella. This pendant position is constantly observed 

 in younger specimens, and seems to be a natural conseqvience of their compara- 

 tive thinness and slenderness; but in jDroportion as the animal grows larger, they 

 increase considerably in thickness, especially toward the base and along the outer 

 or upper keel of each arm, while at the same time, the free margins spread and 

 widen, becoming folded and lobed to such an extent, that each margin ajipears like 

 a ruffled curtain, with innumerable fringes along the whole outline. While this is 

 going on, the open cylinder leading to the main cavity, in the young, becomes 

 gradually more and more distinctly quadrangular (PI. XP. Fig. 17) ; the furrow along 

 the middle of the jirolongation of the angles of the mouth, which is at first very 

 broad and shallow, grows comjoaratively deeper and also narrower; the tAvo sides 

 of each oral appendage closing more and more upon themselves ; and by the time 

 the Aurelia has reached dimensions of about two inches, the oral aperture itself 

 is almost constantly closed, by the approximation of two of its opposite sides. This 

 tendency to closing reaches its maximum in the adults, in which the combined 

 edges of two adjoining arms are brought into linear contact with the combined 

 edges of the ojaposite arms, so that instead of a square opening, leading into the 



