166 ECHINODERMS OF THE BRITISH ISLES 



10. Disk largt^ and flat, witli iiakofl scales ; arms stout, straight, 



with a distinct keel along ventral mid-line, often also along 



dorsal mid-line .... VIII. OphiodiitonidcB 



Disk usually small, mostly with spines or granules ; arms not 



keeled, often moniliform, usually much curved . .11 



11. Disk with tlistinct, naked scales ; arms slender, not moniliform ; 



arm spines few, short, conical, equal-sized ; first ventral plate 

 often divided (genus Amphiactis) . V. Ophiactidce (partly) 



Disk usually with spines, stumps or granules, more or less 

 completely covering the scales ; arm spines usually numerous, 

 long and slender, of unequal length, the uppermost the 

 longest ...... IV. Ophiacanthidce 



(As the distinguishing character used under section 2 of this key : 

 spines appressed or erect, may not always be so distinct as to be 

 indubitable {e.g. Ophiochiton, some Ophiolepidae), it is advisable, 

 especially for the beginner, to whom the general appearance of 

 the various families is not yet famihar, to test the decision, 

 whenever it seems doubtful, by comparison with the special 

 descriptions and the figures. To make the key to the families 

 of the Ophiurae so complete as to exclude mistakes by the 

 beginner would necessitate the use of anatomical characters 

 observable only on dissection, which would not be desirable in a 

 work like the j^resent one.) 



I. Family Ophiomyxid^ 



A thick, naked skin covers both disk and arms, concealing the 

 scales and plates ; even when dried the plates are only indis- 

 tinctly observable. Radial shields more or less rudimentary. 

 Scales of disk mostly very fine, glassy, elliptical disks, with 

 few small holes or none, observable only by microscopical 

 examination of the skin. Dorsal plates of arms rudimentary ; 

 ventral plates well developed. Mouth papillae present ; teeth or 

 tooth papillae. Second pair of tube-feet outside or inside the 

 mouth edge. Spines erect, covered with skin, but when dried or 

 the skin removed, seen to be more or less glassy and thorny. 



The skin of the disk is exceedingly brittle, and simply dissolves 

 on specimens lying only a few minutes in the air before being 

 preserved. Specimens in the collections therefore are very often 

 in a poor state of preservation. 



Development and biology of the European forms of this 

 family unknown. Some littoral forms of the sub-antarctic regions 

 are viviparous, but not hermaphrodites, in contradistinction to 

 most other viviparous Ophiuroids. 



