BEMAEOSTOMEiG -AT IM. I.I.I v. 625 



convoluted ridges on the subumbrella floor of the 4 genital cavities. '1'he bases of the genital 

 ridges are beset with numerous, small, gastric cirri. 



The gelatinous substance of the disk is ut" a translucent milky-white or yellowish-brown; 

 spermaries usually slightly pink. In old individuals the gonads in both sexes art- white. 



Common from Greenland to tin \Vist Indies. At Kastport, Maine, it is mature in Sep- 

 tember, and at Tortugas, Florida, in May. 



The American medusa is closely allied to AurAlia nuritii of Europe and is at most 

 merely a variety of the latter. It may possibly ditfcr from its iMiiopean representative in 

 the thickness and rigidity of the mouth-arms, which are very broad at their bases and often 

 complexly convoluted at their free edes, but I have seen these same characters in Aurellia at 

 Naples and consider the American and European medusa: to be identical. 



A very complete description and numerous figures of the American medusa are given 

 by L. Agassiz, 1860-62, Coin. Nat. Hist. U. S., vols. $ and 4. 



Development. The ova are dehisced from the gonads into the interradial grooves trom 

 which they enter the median gutter of the mouth-arms and are In u- in.iined in small pouches 

 near margins of free edges of mouth-arms and finally set tree in the planula stage. Minchin, 

 1889, gives a good description of these brood-pouches. Segmentation total and unequal, and a 

 blast ula is formed which has a large, central blast ocu-l. According to Hyde, 1894, the gastrula 

 may be formed in either one ot two different ways: ( l ) by the imagination ot a small part ot 

 the blastula wall combined with the ingression of numerous cells from various pans oft be vv;ill 

 of the blastula; (2) by inv agination of the wall of the blastula, aided only occasionally by the 

 ingression of cells from the blastula wall. According to Smith, 1891, however, the gastrula is 

 formed from a small invaginated region in the wall of the blastula, trom which there develops 

 a single, continuous layer of cells, which layer finally completely tills the cleavage cavitv, thus 

 giving rise to a 2-layered embryo with an open blastopore. Smith denies that this process is 

 aided in the least by the ingression of cells from the wall of the blastula into the hlastocuel. Ik- 

 finds, indeed, that a few cells are occasionally seen to wander into the blastula cavity, but these 

 always degenerate without taking any share in the formation of the entoderm. lluse varia- 

 tions in the mode of forming the gastrula have been seen in other Scyphomedusae, having been 

 observed by Conklin in J.inuche, and Hyde and McMurrich in Cyanea. The blasiopou 

 then closes and the entoderm becomes a closed sac entirely enveloped by the ectoderm. The 

 larva then becomes ciliated and swims actively about as a pear-shaped planula. which soon 

 attaches itself to the bottom by the wide anterior end. A crater-like depression (oesophagus) 

 formed of ectodermal cells then appears at the narrow (now the upper) end of the animal, and 

 this presses down upon the entodermal sac. The first pair of radial stomach-pouches is formed 

 from the entodermal sac, while the second pair is formed, at least partially, from the ectoderm 

 of the cup-like depression. The mouth breaks through and 4 tentacles appear. The larva 

 then has 4 interradial, longitudinal, partial septa each formed ot a fold of the entoderm sup- 

 ported by a central shelf of gelatinous substance. These septa extend from the margin ot the 

 mouth to the lower end of the stomach-cavity. They form the 4 primary, gastric filaments 

 of the future ephyra, and there are no septa in the central stomach ot the medusa. 



As we have stated, it appears from the researches of Gone and ot Hyde that the two 

 original pairs of stomach-pouches are derived alternately from the ectoderm of the (esophagus 

 and from the entoderm of the primitive stomach, although Hyde shows that the lower aboral 

 floor of the 2 oesophagus pouches is formed at least partially horn entoderm. Through 

 division of the 4 original stomach-pouches we have finally 24 pouches, 10 entodermal and 14 

 mainly ectodermal, as follows: 6 diammetrically opposite peiiadial, 4 interradial. and the 4 

 connecting adradial pouches are ectodermal. 90 apait fiom these the 6 perradial and their 4 

 adjacent adradial pouches are entodermal. (See Gotte, 1887.) 



One must remember that R. P. Higelow, 1900, finds that the 4 primary stomach-pouches 

 ofCassiopfa x.unn, hana are wholly entodermal, and I !ad/i, 1907, finds that this is also the case 

 in C/irvsaora. Moreover, according to I lad/i there is no ectodeimal imagination in Cliryxiora 

 to form the mouth, but on tin contrary the throat is ^c.i^innl.J and lined with entoderm. 

 The larva of .lin,-lli,i becomes a scyphostoma which finally attains a height of about 5 mm. 

 and acquires 8, 16, and finally 24 long tentacles. The ephyr:i- aie developed tlinni<;h stiobili- 

 zation of the scyphostoma. As many as 13 annular const unions may develop below the zone of 



