68 BULLETIN 76, UNITED STATES NATIONAL MUSEUM 



Genus HYDRASTERIAS Sladen 



Plate 27, Figures 7, 8 



Hydraslerias Sladen (subgenus), Challenger Asteroidea, 1889, p. 581. Type Asterias 



(Hydrasterias) ophidion Sladen. — Fisher, 1923, p. 251. 



Pedicellasler part Pekrier, Exp. Sci. Travailleur et Talisman, 1894, p. 100. — Ludwig, 



Mem. Mus. Comp. Zool., vol. 32, 1905, p. 216. 



Diagnosis. — Typical Pedicellasterinae differing from PediceUaster in the entire 

 absence of actinal plates and in having only one kind of crossed pedicellariae. Infero- 

 marginal plates juxtaposed to adambulacral plates; inferomarginal spinelet not 

 conspicuously larger than the superomarginal and abactinal spinelets which are small, 

 slender, sharp, and isolated; carinal and superomarginal plates + form; infero- 

 marginals same, but with ventral lobe more or less suppressed; dorsolateral skeleton 

 somewhat irregular, as a rule, with two or three longiseries of large meshes, the plates 

 either three or four lobed, and in well-grown specimens connected by slender, elliptical 

 oblong secondary ossicles; first pair of postoral adambulacral plates entirely separated 

 interradially ; adambulacral spines two or three in a transverse series; crossed pedi- 

 cellariae rather thickly scattered over papular areas and on plates but not on spines, 

 all of one kind, without enlarged terminal teeth as in PediceUaster; straight pedicel- 

 lariae small and confined to adambulacral and mouth plates; or larger, blunt, spatulate 

 denticulate ones may be present in axillary region; gonads 20 open on side of ray just 

 above the superomarginals and hence on lower edge of the abactinal region. 



Remarks. — Hydrasterias was first described by Sladen in 1889 as a subgenus of 

 Asterias. Apparently its true relationship has not been suspected, because all species 

 subsequently assigned to the group are in no way closely related. Hydrasterias 

 diomedeae Ludwig 21 is the six-rayed fissiparous young stage of Sclerasterias alexandri 

 His Hydrasterias species is a young stage of the same, or another Sclerasterias. Hydra- 

 sterias richardi Perrier, 22 the specimens of which are very small, is probably also a 

 six-rayed stage of a five-rayed adult asteriid. 



The history as shown by Sclerasterias alexandri and S. euplecta (Fisher) is briefly 

 this: The young at first develop six rays. They divide into two halves of three rays 

 each. Three new rays are regenerated. There is evidence that a second division 

 may take place nearly at right angles to the first (since a resulting animal has one 

 long and two short rays, while another in the same haul has two long and one short). 

 After the last division, when the animal is still small, regeneration produces a five- 

 rayed condition, which results in a symmetrical five-rayed adult. 



This is a case of asexual reproduction inasmuch as one egg may give rise to a num- 

 ber of adults, as in Aurelia. 



Hydrasterias verrilli Fisher 23 although superficially resembling Hydrasterias 

 ophidion has four rows of tube-feet, a narrow oral angle, with several small pairs of 

 postoral adambulacral plates in contact interradially, and apparently belongs in the 

 Asteriinae. It is now the type of Tarsastrocles Fisher. 



» In H. seiradiata (Perrier) and H. improvisa (Ludwig) which I have examined, males in both cases 



" Mem. Mus. Comp. ZoSl., 1905, p. 242, pi. 34, fig. 204; pi. 35, fig. 206. 



» Eip. Sci. Travailleur et Talesman, 1894, p. 109, pi. 9, fig. 4. 



" Starfishes of the Hawaiian Islands, 1906, p. 1106, pi. 41, figs. 3, 3o, 6. 



