TRACHYMEDUS.E — AGLAURA. 



399 



the small ciliated ectodermal cells. About 26 hours after the egg has been laid, 2 tentacles, 180° 

 apart, grow out from the sides of the larva near the hinder end. The entoderm takes the initia- 

 tive in the formation of these tentacles, the central cells dividing and pushing the ectoderm out- 

 wards. A central cavity then appears in the entoderm of the body of the larva and the conical 

 hypostome forms between the tentacles. Then two more tentacles, 90 from the first pair, grow 

 out and the mouth breaks through. The larva then becomes an actinula with a globular, 

 rounded body and conical-cylindrical neck, and with 4 tentacles, 2 long and 2 short. The ecto- 

 dermal surface is ciliated throughout. The mouth is now a simple, round opening. Soon 2 

 more tentacles grow out, 180 apart, quite near the bases of the secondary tentacles and these 

 are soon followed by 2 more tentacles in the same quadrants but near the bases of the 2 primary 

 tentacles. A pair of lithocyst-clubs then appears between the 4 youngest tentacles and 

 another pair of lithocyst-clubs develops 90 from the first. The later development of tentacles 

 appears to be quite irregular. The mouth becomes laterally compressed so as to form a slit 

 having 2 lips 180 apart. 



The bell begins to grow out after the lithocyst-clubs have appeared. It begins in 8 lateral 

 expansions of the body wall which soon form a collar-like ring around the body. 8 entodermal 

 radiations from the central stomach enter these 8 lateral expansions and constitute the 8 



Fig. 250. — Aglaura hemistoma, after Haeckel. 



Fig. 251.— Aglaura hemistoma, abnormal medusa showing lateral fusions between gonads. From life, by the author, Naples 



Zoological Station. January and February, 1908. A, with 3 fused and 2 separated gonads. B and C, with 8 fused 



gonads forming umbrella-like expansion around manubrium. 



radial-canals of the medusa. This formation of the bell occurs when the larva has 12 tentacles 

 and 4 lithocyst-clubs. There are at first only 2 lips; later a second pair develops, forming a 

 cross with the first. It appears that the actinula larva is converted secondarily into a medusi- 

 form adult and the bell forms only after the tentacles and lithocyst-clubs have made their 

 appearance. 



Aglaura hemistoma, in common with many other medusae of world-wide distribution, 

 gives rise to many imperfectly distinguished varieties or local races. I have called certain of 

 these "varieties," more out of respect to their authors than because I believe they represent any 

 natural distinctions. Indeed, out of any collection of a few hundred specimens of A. hemistoma 

 it would be possible to construct an even greater number of "varieties." I agree with Vanhof- 

 fen, 1902, that A. hemistoma is universally distributed throughout all tropical and warm oceans 

 and that its so-called varieties are of no greater importance than are the races of A ureha aurita. 



The extremes of the series are represented by Aglaura "laterna" and A. " octagona 

 with their long, narrow peduncles, and by A. nausicaa with its very short peduncle. A. 

 prismatica, hemistoma, vitrea, and radiata form an intergrading series, the first-named having 

 short and the last moderately long peduncles. The peduncles of the various individuals of 

 every swarm of Aglaura are, however, remarkably variable in length (plate 49, figs. 3 to 5) and 

 I am inclined to regard this character as of no worth in distinguishing species. I have seen 20 

 individuals from a single surface-tow at Tortugas, Florida, in which the peduncles ranged from 

 one-fourth to 1.5 times the length of the bell-radius, and yet they appeared alike in all other 

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