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XX. — Note on a Remarkable Alcyonarian, Shcderia * mirabilis 



g. et sp. n. 



By Professor J. Arthur Thomson, M.A. 



(Bead November 18, 1908.) 



Plate XVI. 



A collection of Alcyonarians made by the ' Investigator ' in the 

 Indian Ocean included a specimen from the Andamans which is- 

 certainly one of the most remarkable of the many interesting 

 representatives of this sub-class that have been discovered within 

 recent years. It is a cup-like colony, with a large retractile poly- 

 parium. The cup is 45 mm. in height by 55 mm. in maximum 

 diameter, and it is continued into a basal wisp (19 mm. in length), 

 which, however, shows no attaching disk. The specimen gave 

 indication of having been imbedded in the mud up to about the 

 maximum diameter of the cup. 



General Structure. — The most striking peculiarity of this 

 Alcyonarian is that the whole of the polyp-bearing portion is 

 retracted within the exceedingly substantial, densely spinose cup, 

 the circular mouth of which is about 30 mm. in diameter, and 

 shows the tips of numerous finger-like polyp-bearing lobes or 

 branches. It seems quite likely that the mouth of the cup was 

 capable of more complete closure, and, on the other hand, that the 

 retracted polyparium was capable of considerable protrusion. 



A longitudinal median section of the single specimen shows a 

 dome-shaped fleshy centre, or thalamus, from the margins and 

 summit of which most of the numerous finger-like polyp-bearing 

 lobes arise. Some of them, however, are attached to the inner wall 

 of the cup at different levels. The central dome, it should be 

 noted, rises quite freely in the middle of the cup; its diameter is 

 greater than half the maximum diameter of the cup. The arrange- 

 ment of the polyp-bearing lobes may be compared to the distribu- 

 tion of carpels and stamens in the flower of some of the Rosacea?, 

 in which the former are disposed on a dome-shaped central thalamus, 

 and the latter on several whorls on the inner wall of the " calyx- 

 tube." Or, again, the central region of our specimen may be com- 



* I have named this type in honour of Professor Th. Studer, of Bern, who has 

 contributed so largely to our knowledge of Alcyonaria. 



