14 Spong. 
XVII. spongia:. 
(iv) Skeletal System. 
(a) General Accounts. —The Bath-Sponge; Seurat (52) p. 258. 
(b) Material of the Skeleton. —The nature and minute structure 
of calcareous spicules, studied in the large monaxons of Leucandra aspera. 
The spicules show a very distinct finely alveolar structure when heated, 
but do not at the same time increase in thickness, as do siliceous spicules 
under similar conditions, so that the resulting distinctness of the alveolar 
structure cannot be attributed to the enlargement of spaces originally 
present, but possibly to a fusion of minute cavities into laTger, visible 
alveoli. The spicules show no trace of an axial thread. The so-called 
sheath which remains when the spicules are dissolved in strong potash 
appears not to consist purely of organic matter, but is composed also 
of inorganic material in some form, possibly CaC0 3 ; Butschli (9). 
The nature and minute structure of siliceous spicules, studied in the 
macroscleres of Geodia placenta and Tethya lyncurium. The siliceous 
substance is amorphous silica, corresponding to mineral opal. Heating 
the spicules produces a finely alveolar system of minute closed spaces, 
probably present in the normal unaltered condition, but too fine to be seen 
until the alveoli have been enlarged by the action of heat in converting 
into vapour the water contained in them. The stratified structure of the 
spicule is further evidence for this view. The axial thread shows the 
reactions characteristic of albuminous substances (pp. 262-267), and in 
the normal condition is firm and brittle, but becomes softened when 
isolated by hydrofluoric acid. A certain amount of organic matter is 
contained also in the siliceous substance of the spicule. The spicules 
of Tethya are enveloped by a great number of richly branched and anasto¬ 
mosing bands, which stain deeply with dahlia, and evidently represent the 
silicoblasts (p. 269, text figg. a & 6), [cf. Maas (37) n, b, iv, c infra] ; 
and in spicules in which the axial thread is deeply stained and shrunk, 
there can be seen not infrequently bodies similar to cells, generally much 
branched, at regular intervals between the axial thread and the wall of the 
axial canal (p. 271, pi. xxi, figg. 10-12) ; Butschli (9). 
The nature of the material of the skeletons of Miocene siliceous sponges; 
Malfatti (40) pp. 276-279. 
(c) Morphology, Development and Nomenclature of 
Skeletal Elements. —Sponge-spicules not to be regarded as inter¬ 
mediate between true crystals and living structures; criticism of the 
“ biocrystal ” theory of Haeckel; Butschli (10) p. 81. 
Spiculation of Hexactinellids in general and of Euplectella in particular, 
pp. 42-57. T-shaped triactins termed Thetactins , p. 44 (the name should 
more correctly be Tauactins ; Lendenfeld in review).—The first spicules 
formed in the larvae of Vitrolhda fertile and Leucopsacus orthodocus 
(Hexactinellids) are stauractins, which are replaced in the adults by 
pentactin derm alia, pp. 162 & 193 footnote and 246 footnote. Develop¬ 
ment of the liexasters, especially of the floricomes, pp. 52 & 53, pi. v, 
figg. 29-31, and the graphio'comes, pp. 53 & 54, figg. 32-35, of Euplectella 
marshalli. Each hexaster begins its development as a liexactin, imbedded 
in a syncytial scleroblast-mass, a body of protoplasmic substance inclosing 
a number of crowded nuclei, pp. 192-200, see also pp. 75-77; Ijima (29). 
Structure of the skeletons of Miocene siliceous sponges ; Malfatti (40) 
pp. 274-276.—Peculiar enlargements and dilatations of the axial canal of 
the trisenes of Isops canaliculata and the oxeas of Topsentia glabra ; 
Topsent (59) pp. 336 & 347, pi. xiv, figg. 5, ?! & 6, o.—Development of 
Litliistid desmas; Schrammen (50) pp. 23 & 24. 
