74 ASTERIAD^E. 



description we have given does not tend to corroborate it ;" 

 — and adds, " We must confess ourselves unable to offer 

 more than mere conjecture as to the use of this singular 

 structure. If the fluid contained in the feet and their 

 vessels be sea-water (either pure or with an admixture 

 of organic particles), which is probable from its chemical 

 composition, may it not be introduced and perhaps again 

 discharged through the pores of the disk and the calcareous 

 tube, the porous disk serving as a sort of filter to exclude 

 impurities V I do not agree with either of these expla- 

 nations. My friend, Dr. Coldstream of Leith, has sug- 

 gested one much more satisfactory to my mind, and of a 

 more philosophical character, namely, that this singular 

 column is the analogue of the stalk of the Crinoid Star- 

 fishes. We see the tubercle indicating it in the Urchins, 

 which are free ; but we find no traces of it in the Cri- 

 noidese or in the Ophiuridse, the former of which are cer- 

 tainly fixed animals in the young state, the latter pro- 

 bably so. 



The centre of the true Starfish's interior is occupied by 

 the stomach, which is thin and membranous, rounded, and 

 slightly lobed, and which has but one external orifice ; 

 from it branch out into each arm two caeca, which are of 

 a similar texture, and very much pinnate and ramified. 

 They, as well as the stomach and the lining membrane of 

 the body, are covered with very minute vibratile cilia. 

 The appearance of these cseca I regard as a first step 

 towards the separation of the respiratory organs from the 

 digestive. Higher up among the Echinodermata we find 

 those cseca almost separated from the digestive system, 

 and mainly appropriated to the purposes of respiration. 

 Above the stomach in some species are seen pyriform 

 sacs, the special uses of which arc unknown. On each 



