SPONGES 



43 



Skeleton. 



may in some cases be due to the continued growth of 

 several endodermal folds towards the exterior, with a 

 corresponding absorption of the mesoderm and ectoderm 

 which lie in the way, till the folds penetrate to the ecto- 

 derm and open at the exterior, thus giving rise to excurrent 

 openings, which are not readily distinguishable from pores. 

 At the same time the original osculum closes up and 

 entirely disappears. Lipogastrosis, on the other hand, 

 may be produced by the growing together, of the roots of 

 the choanosomal folds, thus reducing the paragastric cavity 

 to a labyrinth of canals, which may easily be confounded 

 with the usual form of excurrent canals. While in some 

 sponges the original oscule is lost, in others secondary 

 independent openings, deceptively like oscules, are added. 

 This pseudostomosis is due to a folding of the entire sponge, 

 so as to produce secondary canals or cavities, which may 

 be incurrent (vestibular) or excurrent (cloacal), the opening 

 of the latter to the exterior being termed a false oscule 

 or pseudosfome. The faulty use of the term oscule for 

 what is neither functionally nor morphologically a mouth 

 is here obvious, for in one sense the oscule is always a 

 pseudostome ; it would be better if the term pseudoproct 

 could be substituted. 



Skeleton. — All sponges, except three or four genera be- 

 longing to the Myxospongise., possess some kind of skeletal 

 structures. They may be either calcareous or silicious or 

 horny soleres, the latter usually having the form of fibres, 

 which sometimes enclose silicious needles (spicules) or 

 foreign bodies introduced from without. Foreign bodies 

 also contribute to the formation of the skeleton of some 

 silicious sponges, and occasionally form the entire skeleton, 

 no other hard parts being present. 



Mineral Mineral scleres usually occur in the form of spicules. 



spicules. The spicules of calcareous sponges consist of carbonate of 

 lime, having the crystalline Structure and other properties 

 of calcite (^p). Each spicule, so far as its mineral com- 

 ponent is concerned, is a single crystal, all the molecules 

 of calcite of which it is built up being similarly oriented. 

 On the other hand, its form and general structure are 

 purely organic. Its surfaces are always curved, and usually 

 it has the form of a cone or combination of cones, each of 

 which consists of concentric layers of calcite surrounding 

 an axial fibre of organic matter, — probably of the same 

 nature as spongiolin or spongin, the chief constituent of 

 the fibres of horny sponges. A thin layer of organic matter, 

 known as the spwule sheath, forrhs an outer investment to 

 the spicule and is best rendered visible as a residue by 

 removing the calcite with weak acid. Silicious spicules 

 consist of colloid silica or opal, and hence can be distin- 

 guished from calcareous by having no influence upon polar- 

 ized light. Structurally the two kinds of spicules present 

 no important diflference. The spicules of different sponges 

 differ greatly both in form and in size. They may be 

 conveniently divided into two groups, — minute or flesh 

 spicules, which usually serve as the support of a single cell 

 only {microscleres), and larger or skeletal spicules, which 

 usually contribute to the formation of a more or less con- 

 sistent skeleton {megascleres). The distinction is not one 

 that can be exactly defined, and must so far be regarded 

 as of a provisional nature. There is usually but little diffi- 

 culty in applying it in practice, except in some doubtful 

 cases where large spicules do not form a continuous skeleton, 

 or in others where flesh spicules appear to be passing into 

 those of larger size. It is indeed highly probable that all 

 large spicules have originated from flesh spicules {12). 



(1) Monaxon Biradiate Type {rhaidus). — By far the 

 commonest form is the oxea, a needle-shaped form pointed 

 at both ends and produced by growth from a centre at the 

 same rate in opposite directions along the same axis. It 

 is therefore uniaxial and eguibiradiate (fig. 1 2 a). (2) Mon- 



Mega- 

 scleres. 



axon Uniradiate' Type (stylus). — By the suppression of one 

 of the rays of an oxea, an acuate spicule or stylus results 

 (fig. 12 6). (3) Triaxon Triradiate Type. — Linear growth 



Fig. 12.— Typical megascleres. a, rhabdus (monaxon diactine); 6, stylus 

 (monaxon monactine) ; c, triod (triaxon triactine) ; d, ealthrops (tetraxon 

 tetractine) ; e, triaxon hexactine ; /, desma of an anomocladine Lithistid 

 (polyaxon); g, sterraster (polyaxon); h, radial section tlirough the outer 

 part of g, showing^ two actines soldered together by intervening silica, the 

 free ends terminating in recui'ved spines and the a.xis ti'aversed by a central 

 fibre. 



from a centre in three directions inclined at an angle of 

 120° to each other gives rise to the primitive form of tri- 

 radiate spicule so eminently characteristic of the calcareous 

 sponges, but by no means confined to them (fig. 12 c). (4) 

 Tetraxon Quadriradiate Type (Galthrops). — Growth from a 

 centre in four directions inclined at about 110° to each 

 other produces the primitive quadriradiate form of the 

 Tetractinellida and of some calcareous sponges (fig. 12 d). 

 (5) Sexradiate Type. — Growth in six directions along three 

 rectangular axes produces the primitive sexradiate spicule 

 of the Hexactinellida sponges (fig. 12 e). (6) Multiradiate 

 ^ype.^ Extensions radiating in many directions from a 

 centre produce a stellate form (fig. 12/). "(7) Spherical 

 Scleres. — Concentric growth of silica about an organic 

 particle produces the sphere, which occurs as a reduction 

 of the rhabdus in some species of Poecillastra, or as an 

 overgrown globule (fiesh spicule) in Gaminus. 



Usually conical, the spicular rays often become cylindrical ; usu- Uniaxial 

 ally pointed iftxeate) at the ends, they are also frequently rounded type, 

 off {strongylate), or thickened into knobs (tylotate), or branched 

 Their growth is not always rigorously confined to a 



Fig. 13.— Modifications of monaxon type, a, strongyle ; h, tylote ; c, oxea ; d, 

 tylotoxea ; e, tylostyle ; /, style ; g, spined tylostyle ; h, sagittal triod (a 

 triaxon form derived from the monaxon) ; j, oxytylote ; fc, anatrisene ; I, pro- 

 trigene ; m, orthotrisene ; n, dichotrisene ; o, centrotrijene ; p, amphitrisene 

 (this is triehocladose) ; g, crepidial strongyle (basis of Bhabdocrepid Lithistid 

 desma) ; r, young form of Rhabdocrepid desma, showing crepidial strongyle 

 coated with successive layers of silica ; s, Bhabdocrepid desma fully grown. 

 The dotted line through the upper iigures marks the origin of the actines. 



straight line : frequently they are curved or even undulating. They 

 are also liable to become spined, either by mere superficial thicken- 

 ing or by a definite outgrowth involving the axial fibre (fig. 13 g, h). 

 The rhabdus if pointed at both ends is known as an oxea (fig. 

 13c); if rounded at both ends as a strongyle (fig. 13b); if knobbed 



