90 



Guide to the Invertelrata. 



Echino- 

 denna. 



GALLEEY 



VIII. 

 East Side. 



Wall cases 

 15 18, 

 Table- % 

 cases 

 73-78. 



arms of tlic brittle-stars are nearly always five in number, though 

 OpJdactis and Ophiacantha sometimes have from six to eight. 

 As in the starfish, the arms are unbranched, except in the 

 AstrophytidoD, where they fork ten or twelve times, and where 

 tlie numerous branches interlace so as to form a kind of basket- 

 work all round the disc, whence these animals are called Basket- 

 lish or Medusa-head Starfish. A crinoid differs markedly from 

 a sea-urchin, starfish, or brittle-star, in that the mouth faces 

 upwards, or away from the sea-floor ; the anus is also on the 

 upper surface. This position is due to the fact that, so far as we 

 know, all crinoids are at some time of their lives attached by a 

 stalk to the sea-floor or some other object, so that mouth and arms 

 naturally move up to that side of the body furthest from the stalk. 

 This fixed state of existence has also caused the development of 

 arms, five in number, but often forked many times, which arms 

 stretch out from the body on. all sides of the mouth, and contain 

 extensions of the nervous, blood-vascular, water- vascular, and 

 generative systems. 



I . — HOLOTHUEOIDEA . 



Sea-cucumbers are represented as fossils only by the spicules 

 and minute plates formed in the skin. These are known from 

 Carboniferous times. Spicules of Cucumaria from the Pliocene of 

 St. Erth, Cornwall, and plates of Psolus from Scotch Glacial beds, 

 are exhibited. 



II. — ECHINOIDEA.. 



This class contains those Echinoderraa in which the body is 

 completely surrounded by a "test," composed of closely-fitting 

 calcareous plates arranged in zones, of which there are usually 

 ten pairs. The mouth is always situated on the lower surface, 

 but the anus may open either at the centre of the upper surface, 

 Wall cases opposite the mouth, on the margin, or on the same side as the mouth. 

 The test of the common '* sea-urchin" found on the English 

 shores {Echinus esculeidus) is composed of twenty rows of plates 

 placed in vertical series, together forming an almost spherical case. 

 The plates are of two sizes: the smaller, or " ambulacral plates," 

 each consist of three others, fused together so intimately, that 

 the composite character of the plate can often only be recognized 

 owing to the presence of three pairs of small pores. 



The whole "test" is covered with tubercles on which, in the 



Holothu- 

 rians. 



Table-case 

 78. 



Echi- 

 noidea- 

 Sea- 

 urchins 



Table- 

 cases 

 76 78, 



