CORONATjE — ATOLL A . 



563 



AtoUa bairdii Fewkes. 



AtoJIa bairdii, Fewkes, J. W., 1886, Report Commiss. Fish and Fisheries U. S. A. for 1884, p. 936, plates 1-3, 4 figs.; 1888, 

 American Journ. Sci., ser. 3, vol. 35, p. 172; Ann. Mag. Nat. Hist., ser. 6, vol. I, p. 253; 1889, Report Commiss. Fish 

 and Fisheries U. S. A. for 1S86, p. 530.— Agassiz, A., 1888, Bull. Mus. Comp. Zool. at Harvard College, vol. 15, p. 131, 

 fig. 427.— Vanhoffen, 1892, Ergeb. der Plankton Expedition, Bd. 2, K. d., p. 16, taf. 4, fign. 3-9; 1902, Wissen. Ergeb. 

 dcutsch. Ticfsee Exped., Dampfcr I'aldivia, Bd. 3, Lief. I, p. 9; 1906, Nordisches Plankton, Nr. 11, p. 44, fign. 4, 5. — 

 Maas, 1897, Mem. Mus. Comp. Zool. at Harvard College, vol. 23, p. 79; 1904, Result. Camp. Sci. Prince de Monaco, 

 fasc.28,p.49, planche4, figs. 29-34: planche 5, figs. 38-43. — Brown v., 1906, Trans. Linnean Soc. London, ser. 2, Zool., vol. 

 10, p. 179. 



The medusa is 30 to 144 mm. in diameter, disk quite flat. Central lenticular part of 

 exumbrella somewhat more than half as wide as entire medusa. It is flat aborally and sur- 

 rounded by a recurved rim, the periphery of which displays about 20 to 22 slight notches, 

 which are one less numerous than the tentacular pedalia in whose radii they tend to lie. Central 

 lens separated from peripheral part of exumbrella by a deep, broad, annular groove, slightly 

 over-arched by the recurved rim of the central lens itself. The peripheral zone of the exum- 

 brella includes all that part of bell which lies centrifugal from the annular groove. It may 

 in turn be divided into 4 zones: (i) The innermost zone is a simple, smooth-surfaced, annular 

 ridge bounded on the inner side by the deep annular groove and on the outer by a very slight, 

 reddish-colored groove which separates it from (2), the zone of the tentacular pedalia. There 



Fig. 357. — AtoUa bairdii, after Maas in Result. Camp. Sci. Albert i^*" Prince de Monaco. 

 A, and B, side views of the medusa; C, marginal canal-system and ring-muscle (cut across). 



are about 18 to 24, usually 22, pedalia, one in the radius of each tentacle. These pedalia are 

 thick ridges separated one from another by shallow radiating furrows, which do not cut very 

 deeply into the gelatinous substance, so that the tentacular pedalia form a thick, continuous, 

 gelatinous zone. Each pedalium bears a short, solid tentacle, usually carried recurved upward 

 and provided with strong, longitudinal muscles upon its subumbrella side. 



The third zone is that of the pedalia of the sense-organs and lies immediately centrifugal 

 to the zone of the tentacular pedalia. These pedalia of the sense-organs alternate in position 

 with the equally numerous, tentacular pedalia. They are lower than the tentacular pedalia 

 and are separated from one another by deep, wide, radiating furrows which occupy the radii 

 of the tentacles. Sense-organs very small with 2 swollen regions upon the ventral (subum- 

 brella) side of the bulb where one finds thickened, sensory ectodermal epithelium. There 

 is no ocellus and there are no pigment spots either in the ectoderm or the entoderm, but 

 there is a terminal sac-like swelling which contains a crystalline mass of entodermal origin 

 and which is protected by a wide, scale-like expansion on the exumbrella side of the sense- 

 club. (See VanhofFen, 1902; Maas, 1904.) 



The fourth or outermost zone of the exumbrella consists of long, slender, marginal lappets 

 supported by the pedalia of the sense-organs, and which are twice as numerous as the latter. 



