XIX SYNAPTIDA PHYLOGENY 577 



appearance of the two groups, the epigram is almost justified 

 that " if the Synaptida were not extremely careful they would 

 become Gephyrea." 



This order is represented in British waters by three species 

 of the genus Synapta, which is remarkable for possessing, as 

 ossicles, only the peculiar anchors attached to anchor plates. 

 The present author has dug up the commonest species {S. in- 

 haerens) from its burrows in the sand at low water in the 

 Clyde. These animals seem to seek their food at the surface ; 

 the feather-shaped feelers are used to seize small algae and 

 zoophytes, of which the food apparently consists. If seized, S. 

 i7ihaerens readily amputates the posterior part of the body, whilst 

 the head with its feelers immediately buries itself. The other 

 genera of the order (except Anapta) are characterised by the 

 possession of wheels with spokes as their characteristic ossicle, 

 as the names Trochodota, Troclioderma, Acanthotrochus bear 

 witness. 



The only fossil remains of Holothuroidea consist of isolated 

 ossicles — wheels, gratings, anchors, etc. — which first make their 

 appearance in the Carboniferous limestone and tell us practically 

 nothing of the evolution of the group. From a comparison with 

 one another of the living families, certain conclusions can be 

 drawn. The Aspidochirote feeler and the method of using it 

 recall forcibly the shape and function of the buccal tube-feet of 

 Spatangoidea. It is probably safe to assume that it is the primi- 

 tive form from which the other forms of feeler have been derived. 

 Secondly, the anal respiration and the curious internal madre- 

 porite have been developed in correlation with one another, and 

 are like nothing found elsewhere among the Eleutherozoa. 

 Hence we may with high probability assume a Protoholothuroid 

 stock with shield-shaped feelers but devoid of respiratory trees, and 

 with an external madreporite. From this stock the Elasipoda 

 developed by migrating into deeper water, whilst the Pelago- 

 thuriida sprang from the same root by taking to swimming ; the 

 Aspidochirota constituting the main line. The Dendrochirota 

 were developed from a stock with respiratory trees and internal 

 madreporite — in a word, from Aspidochirota. From them 

 the Synaptida and the Molpadiida have developed as offshoots 

 at different periods through taking to a burrowing life. These 

 relationships are shown by the following diagram :— 



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