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PROCEEDINGS OF THE 



[Dec, 1856. 



the circulating system. Again, there is no structure in any known 

 hydroid comparable to the eight lobular muscular areas which are 

 the true homologues of the muscular bell of Discophora, and like it 

 they are the only organ of swimming, and are employed for no other 

 purpose. The form of the digestive cavity is that which is known 

 in the genus Polyxenia (Eschscholtz,) and with the addition of 

 sexual glands in the form of diverticula, in the family of iEginidae. 



But, before proceeding with such a comparison, another and 

 most important change, which is in progress at that moment of 

 which we are speaking, must be noticed. The long proboscis 

 which has already been observed to shorten before the ejection of 

 the larva, upon attentive observation is now found to be rapidly 

 diminishing in length and increasing in relative width. So rapid- 

 ly does this progress, that within twenty-four hours after ejectment 

 the whole proboscis has frequently entirely disappeared, at least is 

 reduced to a mere button. This vestige, however, sometimes is still 

 visible for a while, and I have even seen it take the form represented 

 PL 7, fig. 31a. But as seen in PI. 7, 32b, the final condition of the 

 digestive cavity is a simple eight-pointed, depressed chamber, with a 

 mouth, which, when open, is a simple aperture with a thickened rim 

 in the lower wall of the cavity. When open, the mouth may have a 

 circular or oval outline, but when closed, 1 have seen it present the 

 cruciform appearance characretistic of many Medusae, represented 

 PI. 7,, fig. 32c. Sometimes even the four corners are a little more pro- 

 minent than at others, and an appearance of four rudimentary lips is 

 thus presented. For a short distance, on all sides about the mouth, 

 the wall of the stomach appears to be a little thicker than in other 

 parts, and especially in a stage just anterior to that represented PI. 

 7, fig. 34, is conspicuously colored with light orange like the pro- 

 boscis of the parent Turritopsis. This simple oral aperture of 

 the larva is ordinarily carried wide agap, like the mouth of iEquo- 

 rea, so that, as m that genus, the water of the sea fills the internal 

 cavity or stomach (whose walls appear to be lined with vibratile 

 cilia,) and it is probably by such means that the larva obtains its 

 nourishment at this period. (Vide PI. 7, ff. 33 and 34.) 



No other remarkable changes occur at this stage, save that just 

 described. The disk, perhaps, only increases a little in relative 

 size, the lobes becoming somewhat larger in proportion to the audi- 

 tory vesicles which hang like drops from them. The Medusa is 

 now about .09 in. in diameter, nearly 2' 7 ', and is of a complete 

 watch-glass form, or rather it has the shape of a shallow glass dish 



