250 THE VOYAGE OF H.M.S. CHALLENGER. 



shapeless genital scale soldered for most of its length, and only leaving a small genital 

 opening near the mouth shield. Sometimes the genital scale is soldered with its 

 neighbouring scale in one, and sometimes it nearly or quite disappears, leaving the inter- 

 brachial space almost wholly filled by the two genital plates. The outer face of the arm 

 bone (fig. 5) makes a near approach to that of Astrojjhi/ton, having the articulating 

 shoulder (4) nearly of the transverse hour-glass form : the great muscle fields and large 

 tentacle sockets (/■) are, however, distinctly Oj^hiui-an. The inner face also (fig. 6) has 

 a similar mixed character, with its large umbo (1) prolonged downwards, and forming a 

 vertical hour-glass. The mouth frames (fig. 4,f) are massive and plain, without grooves 

 or ridges, and the peristomial plate (u) is small, single, rounded and intimately connected 

 with the surrounding parts. The resemblance of the arm bone joints to those of the 

 AstrophytidEe is striking ; but is not carried to other parts of the structure, which is truly 

 Ophiuran, although the loading of the tissues with so much lime gives it a character of 

 its own. 



See Plate XLIII. fios. 4-G. 



a 



Species of Sigsheia not herein described. 



Sigsheia murrhina, Lym. (PI. XLIII. figs. 4-6). 



Sigghcia murrhina, Lyiu., Bull. Mus. Comp. Zool., vol. v., part 9, p. 231, pi. iii. figs. 55-58, 1878. 

 West Indies ; 88 to 422 fathoms. 



ASTROPHYTID.E. 



Astrophytidae are a family in the order of stai-fishes characterised by a more or less 

 sharply defined central disk containing a digestive cavity, simple or much pleated, which 

 has no anal opening, and does not pass into the arms. These, sometimes simple and 

 sometimes ramified, have a central axis composed of jointed, vertebra-like sections (arm 

 bones), each made up of two ambulacral pieces soldered side by side. Their joints consist 

 of a horizontal and a vertical hour-glass-like jirojection fitted one on the other. The axis 

 is covered by a thick skin, under which are plates, generally of an irregular and elementary 

 character ; and there are no spines on the sides of the arms. Each arm bone is pierced 

 by a water tube, destitute of a bulb, and supplying the imperforate tentacle, which is 

 imbedded in the bone itself. The halves of the first two arm bones are swung laterally 

 into the iuterbrachial space and soldered together to form the mouth angle ; and in them 

 are set the mouth tentacle which are watered by a forking tube from the mouth ring. On 

 either side of the base of each arm, above and below, run two stout pieces, the radial 

 shield and genital plate, which are joined at the margin of the disk, and are connected 

 by an adductor muscle. In the lower interbrachial space, close to and parallel with each 



