ARCTIC SEA TO THE WEST OF GREENLAND. 47 



ossicles, whereby it follows that the intermediate spaces are suboval or diamond- 

 shaped, and an apparent diagonal arrangement of calcareous plates is produced. Each 

 of the cruciform ossicles bears a single paxilla situated on the margin of the angle 

 that lies nearest to the centre of the disk. The pedicle of the paxilla is short, usually 

 little more than a tubercle, and bears three or sometimes four comparatively long 

 tapering spinelets ; these diverge at a small angle from one another, and are main- 

 tained in this position by numerous extremely fine muscular fibres, which are attached 

 near the tips, and pass from spine to spine in every direction over the entire area. By 

 this means the foundation is laid for a membranous tissue, which is supported over 

 the body like a tent-cloth by the spinelets, and by which the whole dorsal surface of 

 the animal is covered and concealed. A hollow infradermal cavity is thus formed, 

 wherein the development of the ova and embryos takes place*. Each of the inter- 

 mediate spaces or meshes in the calcareous network of the skeleton has a single 

 puncture in its membrane near the base of the paxilla, which gives passage to a papula 

 of very peculiar form. This remarkable organ, instead of being cylindrical with a 

 simple conical tip, as is usually the case, has its extremity split up into a number of 

 short blind cseca or knob-like branchlets, and is attached to the base of the paxilla, 

 being in all probability non-retractile. 



Each adambulacral plate bears a transverse series of five or six long spines, which 

 are connected together by a membrane, and form a webbed fan that stands at a right 

 angle to the ambulacral furrow. The outermost spine of each comb is double the 

 length and thickness of any of the others, but does not always stand in a line with the 

 rest of the fan-like series, being sometimes nearly midway between its own series and 

 the next succeeding ; the other spines of the " ambulacral " comb are nearly equal in 

 size, the middle ones being slightly longer. The long external spines above named 

 extend about half their length beyond the edge of the ray, and are united to one 

 another by a connecting-tissue, which forms the fan-like fringe that surrounds the 

 entire Starfish ; and this also is the boundary of the secondary or supradermal covering 

 of the dorsal surface before described. The mouth-plates are subquadrate in form ; and 

 the mouth-spines, which are 5 or 6 in number, are webbed together and directed at an 

 angle downwards, the innermost spinelet being the longest and stoutest of the series. 

 From the centre of each mouth-plate, and standing perpendicular to its plane, is a 

 very large stout spine, much thicker and larger than any of the others ; this is articu- 

 lated on a small rudimentary tubercle, and has the tip, which is abruptly pointed, 

 transparent and glass-like. 



The anal aperture is situated at the centre of the inner or true dorsal surface of 

 the animal ; and the madreporiform body, which is circular and more or less arched and 

 granulate or tubercular in appearance, lies in a median interradial line near to the 

 centre, and not more than its own diameter away from the periproct. A moderately 

 wide orifice, surrounded by a circlet of enlarged paxillae, is situated in the centre of 



* For details consult Koren arid Danielssen, Fauna litt. Norv., Heft 2, p. 58; Saxs, Overeigt af Jforges 

 Echinodcrmer, p. 58. 



