SPONGES AND ZOOPHiTTES. 121 



undergone either calcification or silification; and by the self -shaping 

 power of which, the form of the spicule is mainly determined. 



511. There is an extremely interesting group of Sponges, in which 

 the horny skeleton is entirely replaced by a siliceous framework of great 

 firmness and of singular beauty of construction. This framework may 

 be regarded as fundamentally consisting of an arrangement of six-rayed 

 spicules, the extensions of which come to be, as it were, soldered to one 

 another; and hence the group is distinguished as hexiradiate. Of this 

 type the beautiful Euplectella of the Manilla Seas which was for a long 

 time one of the greatest of zoological rarities, but which now, under the 

 name of ' Venus's flower-basket/ is a common ornament of our drawing- 

 rooms is one of the most characteristic examples. Another example is 

 presented by the Holtenia Carpenteri, of which four specimens, dredged 

 up from a depth of 530 fathoms between the Faroe Islands and the North 

 of Scotland, were among the most valuable of the 'treasures of the deep' 

 obtained during the first Deep-sea Exploration (1868), carried on by Sir 

 Wyville Thomson and the Author. This is a turnip-shaped body, with 

 a cavity in its interior, the circular mouth of which is surrounded with a 

 fringe of elongated siliceous spicules; whilst from its base there hangs a 

 sort of beard of siliceous threads, that extend themselves, sometimes to a 

 length of several feet, into the Atlantic mud ( 480) on which these 

 bodies are found. The framework is much more massive than that of 

 Euplectella, but it is not so exclusively mineral; for if it be boiled in 

 nitric acid it is resolved into separate spicules, these being not soldered 

 together by siliceous continuity, but held together by animal matter. 

 Besides the regular hexiradiate' spicules, there is a remarkable variety of 

 other forms, which have been fully described and figured by Sir Wyville 

 Thomson. 1 One of the greatest features of interest in this Holtenia, is 

 its singular resemblance to the Ventriculites of the Cretaceous formation 

 ( 699). Subsequent investigations have shown that it is very widely 

 diffused, and that it is only one of several Deep-sea forms, including 

 several of singularly beautiful structure, which are the existing represen- 

 tatives of the old Ventriculite type. One of these was previously known, 

 from being occasionally cast up on the shores of Barbadoes after a storm. 

 This Dictyocalyx pumiceous has the shape of a mushroom, the diameter 

 of its disk sometimes ranging to a foot. A small portion of its reticu- 

 lated skeleton is a singularly beautiful object, when viewed with incident 

 light under a low magnifying power. 



512. With the exception of the genus Spongilla, all known Sponges are 

 marine; but they differ very much in habit of growth. For whilst some 

 can only be obtained by dredging at considerable depth, others live near 

 the surface, whilst others attach themselves to the surfaces of rocks, 

 shells, etc., between the tide-marks. The various species of Grantia,* in 

 which, of all the marine Sponges, the flagellate zooids can most readily 

 be observed, belong to this last category. They have a peculiarly simple 

 structure, each being a sort of bag whose wall is so thin that no system 

 of canals is required; the water absorbed by the outer surface passing 

 directly towards the inner, and being expelled by the mouth of the bag. 

 The flagella may be plainly distinguished with a l-8th inch objective on 

 some of the cells of the gelatinous substance scraped from the interior of 

 the bag; or they may be seen in situ, by making very thin transverse 



1 See his elaborate Memoir in "Philos. Transact.," 1870; and his "Depths of 

 the Sea" (1872), p. 71. 



