424 



ECHINODERMS OF THE BRITISH ISLES 



The numerous forms belonging to this order are either regarded 

 as forming one famil}', with three subfamiUes, or as forming three 

 families ; which course is to be taken is rather a matter of taste. 

 The distinguishing characters of the three groups being found in 

 the calcareous deposits alone, and thus not of primary morpho- 

 logical importance, it may well be preferable to regard all as 

 belonging to one family. 



I. Family Synaptid^ 



Body elongate, worm-shaped. Calcareous ring often with 

 more than five interradial pieces ; radial pieces often perforate. 

 Small ciliated funnels usually present in the body cavity. No 

 Cuvierian organs. A pair of statocysts at the base of each radial 

 nerve. Calcareous deposits mainly anchors and anchor plates, 

 wheels or sigmoid bodies. 



Key to the subfamilies of the Synaptidoe. 



1. Deposits anchors and anchor plates (rarely absent) 1. Synaptince 

 Deposits wheels or sigmoid boaies ..... 2 



2. Wheels with only 6 spokes ; sigmoid bodies often present together 



with the wheels or sigmoid bodies alone . . 2. Chiridotince 



Wheels with 8 or more spokes ... 3. Myriotrochince 



I. Subfamily Synaptin^ 



Deposits of body wall anchors and anchor plates ; small 

 irregular or ovoid bodies often present. Deposits rarely absent. 

 Small rod - shaped spicules in the ten- 

 tacles. In the larvae and, sometimes at 

 least, also in the young specimens wheels 

 are present. 



Anchor and anchor plate are separate 

 bodies, but always closely associated so as 

 to form apparently a single body (Fig. 258). 

 They serve as locomotory apparatus. They 

 are enclosed by the skin, but on handling 

 their anchors very easily break through 

 the epidermis and thus make the skin 

 exceedingly sticky ; especialh^ in the larger 

 tropical forms the anchors will hold very 

 tenaciously in each other or in what else 

 they get hold of, and it may be difficult 

 enough to separate them from each other 

 and get the specimens preserved in an extended state. It is also 

 usualh' necessary to narcotise the specimens {e.g. with magnesium 



Fig. 258. — Anchor and 

 anchor plate of Labi- 

 doplax Buski, as they 

 he together in the body 

 wall. X 200. (From 

 Danniark's Fauna.) 



