460 Prof. P. M. Duncan and Mr. W. P. Sladen on 



Pteraster militaris (O. F. M.), Miiller and Troscliel. 



Asterias militaris, 0. F. Miiller, Zool. Dan. tab. 131 (excl. textu). 

 Asteriscus militaris, Miiller & Troscbel, Sjst. d. Asteriden, p. 44. 

 Pteraster militaris, Miiller & Troschel, Syst. d. Asterideii; p. 128. 



Coll. Hart : Dobbin Bay, 30 fms. 



This starfish is readily distinguished from its congeners and 

 the majority of other asteroids by the singular fin-like margin 

 surrounding the arms, by the membranous skin which is 

 spread over the upper surface, as well as by the series of 

 webbed spines which stand, in ti-ansverse ranges like fans, by 

 the side of the ambulacral furrow. 



The form of the animal is pentagonal, the upper contour of 

 the body high and arched, and the underside flat. Propor- 

 tion of disk-radius to arm-radius 1 : 2. Each inlerambulacral 

 plate is furnished with five or six long spines, which are con- 

 nected together by a membrane into a webbed comb placed 

 transversely to the ambulacral fiuTOw. The outward spine of 

 each comb is double the length of the others, and extends 

 about half its length beyond the edge of the ray. These long 

 spines are also united to one another by a connecting tissue, 

 and thus form the fin-like fringe which surrounds the entire 

 starfish. The ambulacral spines forming the fan-like comb 

 are nearly equal in length, the middle ones being slightly 

 longer. 



The body-skeleton is composed of a calcareous network, 

 from each of the cross joinings of which proceeds a spine- 

 fasciculus bearing three or four spinelets. The whole dorsal 

 surface of the animal is covered and concealed by a mem- 

 branous tissue supported above the body, like a tent-cloth, 

 by the spinelets, to the tips of which it is attached. A 

 hollow intradermal cavity is thus formed. Neither the anus 

 nor the madreporiform tubercle has any special aperture in this 

 investing membrane ; there is, however, a single large-sized 

 opening, surrounded by a margin of spines, situated nearly 

 over the dorso-central axis. In and out of this aperture Dr. 

 Stimpson has observed currents of water passing, as in the 

 cloaca of a Holothuria^ from which fact he was led to regard 

 the functions of the cavity as subservient to respiration *. 

 MM. Koren and Danielssen, however, have pointed out that 

 this intermediate space between the double dorsal skin fulfils 

 a further and more important purpose by becoming a chamber 

 in which the development of the eggs and embryos takes 

 place t- 



* Stimpson, " Marine luvertebrata of Grand Manan," p. 15, in Sinitb- 

 Bonian Contributions, vol. vi. 

 t Koren and Danielssen, 'Fauna littoralis Norve^nag,' Heft 2, p. 58. 



