40 THE DEPTHS OF THE SEA 



ficient in lime, and are fibrous or cartilaginous in 

 composition. Their scales, too, are thin and mem- 

 branous, their skin soft and velvety. The shells of 

 deep-sea molluscs are as thin and translucent as 

 tissue-paper; and the same is true of some brachio- 

 pods. The test of the echinoderms is often soft, 

 and the armour of the crustacea is merely chitinous, 

 unhardened by deposits of lime. Calcareous sponges 

 are altogether unknown in the depths. This inability 

 to form a hard skeleton — curiously enough this does 

 not apply to corals — is not due to any want of cal- 

 careous salts in the bottom waters. It is known that 

 calcium sulphate, from which animals secrete their 

 calcium carbonate, exists in abundance ; but those 

 animals which dwell on the calcareous globigerina 

 ooze are as soft and yielding as those which have 

 their home on the siliceous radiolarian deposits. 

 Animals which form a skeleton of silex do not suffer 

 from the same inability ; in fact, the deep-sea radio- 

 lanans often have remarkably stout skeletons, whilst 

 the wonderful siliceous skeletons of the hexactinellid 

 sponges are amongst the most beautiful objects 

 brought up from the depths. 



The second peculiarity, for which there seems no 

 adequate reason, is the reduction and diminution in 

 size of the respiratory organs. Amongst the Crus- 

 tacea, the ascidians, and the fishes this is especially 

 marked. The gill laminae are reduced in number and 

 in size ; and the evidence all points to the view that 

 this simplification is not primitive but acquired, being 

 brought about in some way by the peculiar conditions 

 of life at great depths. 



When the first attempts were made to explore the 

 bed of the ocean, it was hoped that the sea would give 

 up many an old-world form ; that animals, known 

 to us only as fossils, might be found lurking in the 

 abysmal recesses of the deep ; and that many a missing 



