ASTEROIDEA OF NORTH PACIFIC AND ADJACENT WATERS — FISHER 109 



terias, as represented by their respective types, are highly restricted and peculiar 

 genera, only remotely related to the species under discussion. 



Asexual reproduction. — The life history of three species of Sclerasterias, namely 

 euplecta, alexandri, and heteropaes, affords an example of an alternation of sexual 

 reproduction with asexual. These species have a definite immature stage, unlike 

 the adult, in which hexamerous, not pentamerous, symmetry is predominant. The 

 crossed pedicellariae are not in circumspinal wreaths but are scattered, as in Pedicel- 

 laster, between the spinelets, which are short, uniform, and two to four to each carinal 

 and two or three to each superomarginal plate. The superomarginals usually carry 

 two flattened spines without attached pedicellariae, and the adambulacrals are dip- 

 lacanthid. The integument is not unusually thick. 



In this stage the sea stars actively divide by splitting into equal halves. The 

 number of these divisions is not known, but is not improbably several. In one 

 instance, in 8. euplecta, a division has taken place nearly at right angles with the plane 

 of the prior fission. Since adult Sclerasterias are almost invariably pentamerous and 

 the majority of the young are hexamerous, a ray is lost somewhere in the process, 

 probably at the last division. 



These dividing young range in size from R 8 to R 20 mm., the latter being unusu- 

 ally large. R 15 mm. is nearer the usual maximum size of fissiparous individuals. 



Asterias richardi of the Mediterranean, two specimens of which I have examined 

 (U. S. Nat. Mus., No 18287, lat. 33° 56' N., long. 22° 56' E., H. M. S. Pola) is prob- 

 ably the young phase of a Sclerasterias. It is at first hexamerous, then pentamerous, 

 and its structure is closely similar to that of young euplecta, alexandri, and heteropaes. 



Sclerasterias nitida i3 Koehler from the vicinity of the Andaman Islands like- 

 wise appears to be a typical juvenile phase of a Sclerasterias, possibly of Sclerasterias 

 mazophora (Alcock and Wood-Mason), Andaman Sea, 120 to 250 fathoms. 



All the specimens which I have examined — probably the great majority of those 

 known — were taken by hempen tangles. Rocky situations, unsuitable for dredg- 

 ing, are favored by these tiny sea stars. The presence of rocks would suggest their 

 use for shelter, especially during the time immediately after divisions. Likewise the 

 critical period, during which the fissiparous, hexamerous young changes to the non- 

 fissiparous pentamerous adult phase, is probably passed in a quiescent state in crev- 

 ices or cavities of the rocks or under stones, which would account for the absence of 

 this stage in collections. But during the growth of new rays the young are probably 

 abroad and feeding, and hence are more readily snared by the tangles. 



The largest series of young stages known is that of S. euplecta, secured\lurmg the 

 Hawaiian cruise of the Albatross in 1902 and briefly commented upon by the writer in 

 1906. 41 



Young phase of S. euplecta. — The following is the list of specimens of the young 

 phase of S. euplecta (pi. 81, fig. 7): 



" Ecbinodorma of the Indian Museum, pt. 6., Astfcries littorales, 1910, p. 176, pi. 19. figs. 2. 3, 4. 

 " Hawaiian Starfishes, p. 1105, pi. 42, Bg. 3. 

 64406—28 8 



