XVIII 



CLYPEASTROIDEA 



547 



suckers, but the first variety are much larger than the second ; 

 they possess a flattened lobed base, and are connected with the 

 ampulla by a double canal. They issue only from the double 

 pores which form the petal. The locomotor tube-feet are small 

 and cylindrical ; they are, as already mentioned, scattered over 

 the whole upper surface of the test, penetrating both ambulacral 

 and interambulacral plates, but all are connected by transverse 

 canals with the radial canals of the water-vascular system. On 

 the under surface they are confined to the neighbourhood of 



Fig. 244. Dissection 

 of Echinarachnius 

 parma. x 1. The 

 oesophagus has 

 been cut through 

 and moved to one 

 side so as to ex- 

 pose Aristotle's 

 lantern. The 



aboral part of the 

 test has been re- 

 in o v e d. go n, 

 Genital organ ; int, 

 intestine ; muse, 

 transverse muscle 

 connecting jaws of 

 adjacent inter- 

 radii ; rect, rec- 

 tum ; siph, siphon ; 

 st, stomach. 



rect 



the ambulacral grooves, which have nothing to do with the 

 ambulacral grooves of an Asteroid, but are due to secondary 

 localisations of the tube-feet, which are here also connected in 

 each radius with a single radial canal. The appearance of a 

 living Echinarachnius covered with a veritable forest of short 

 brown tube-feet is very striking. 1 



The condition of the water-vascular system is to be explained 

 entirely by the peculiar environment of the animal. The demand 

 for specialised respiratory organs is brought about by the habit 

 of living half buried in the sand. Under these circumstances 

 the strain of supplying the needful oxygen is thrown on the 



1 These statements are based on the author's observations of the animal in the 

 Bay of Fundy in 1900. 



