44 



SPONGES 



at both ends as a tylote (flg. 13 h) ; the tylote if pointed at one end 

 IS a tylotoxea (fig, 13 d); the strongyle similarly becomes a strongyl- 

 oxea. These last two forms are with difficulty distinguished from 

 the stylus, which is usually pointed at the end, and strongylate (fig. 

 13/) or tylotate (fig. 13 e) about the origin. A particular case of 

 the cladose rhabdus, but one of the most frequent occurrence, is 

 the iriaene ; in this form one ray of a rhabdus ends in three branches, 

 which diverge at equal angles from each other. The rhabdus then 

 becomes known as the shaft or rhabdome, and the secondary rays 

 are the arms or dadi, collectively the head or dadome of the spicule. 

 The arms make different angles with the shaft : when recurved a 

 grapnel or ariatriasne is produced (fig. 13 k), when projecting forwards 

 aprotriasne (fig. 13 I), and when extended at right angles an ortho- 

 irixne (fig. 13 m). The arms of a trisene may biftircate {dichotrieene) 

 once (jig. 13 »), twice, or oftener, or they may trifurcate. Again, 

 they may extend laterally into undulating lamellae, or unite to form 

 a disk, the trisene character of which is indicated by the included 

 axial fibre. The shaft may also become trifid at both ends, amphi- 

 triasne (fig.. 13 p), and the resulting rays all bifurcate, or the cladome 

 may arise from the centre of the rhabdome, centrotriasne (fig. 13 o). 

 Amongst one group of Lithistid sponges {Rhahdoerepida) the normal 

 growth of a strongyle is arrested at an early stage ; it then serves 

 as a nucleus upon which further silica is deposited, and in such a 

 manner as to produce a very irregularly branching solere or desma 

 (fig. 13 s), within which the fundamental strongyle can be seen en- 

 closed. In such a desma no axial fibre besides that of the enclosed 

 strongyle is fonned. 



Triradi- The chief modification of the triradiate spicule is due to an elonga- 



ate tj-pe. tion of one ray, distinguished as apical, the shorter paired rays 

 being termed lasal, and the whole spicule a sagittal triradiate. The 

 angle included by the basal rays is usually over 120° (fig. 14 a). 



Quadri- Some or all of the rays of the primitive oalthrops (fig. 14 V) may 



radiate 



type. 



Sexradi- 



FiG. 14. — ^Modi&cations of the triaxon and tetraxon types, a, sagittal triradiate 

 or triod ; 6, calthrops ; c, candelabra (a polycladose microealthrops) ; d, a 

 spined microealthrops ; e, 3?etracladine Lithistid desma. 



subdivide into a number of terminal spines candelabra (fig. 14 0); 

 or some or all of them may bifurcate once or twice and finally 

 terminate by subdividing into numerous variously shaped processes ; 

 such a tetracladine desma (fig. 14 e) characterizes one division of the 

 Lithistid sponges. 



By the excess or defect of one or more rays a series of forms such 



ate type, as are represented in fig. 15 arise. In the oxea, which results from 



Fig. 15. — ^Modifications of the triaxon hexactine type, a, dagger ; b, c, two 

 varieties of pinnulus ; d, amphidis]£ ; e, pentactine ; f, staurus ; g, dermal 

 rhabdus. After Schulze. 



the suppression of all rays but two, the sexradiate character is some- 

 times preserved by the axial fibre, which gives off two or four pro- 

 cesses in the middle of the spicule where the defective arms would 

 arise. Let fig. 12 e represent a regular sexradiate spicule with its 

 four horizontal arms extended beneath the dermis of its sponge ; 

 the over-development of the proximal ray and a reduction of the 

 distal ray produce a form known as the dagger (fig. 15a); the 

 suppression of the proximal ray and the development of spines pro- 

 jecting forwards on the distal ray produce the pinnulus (fig. 15 J, c) ; 

 the suppression of both proximal and distal rays gives the staurus 

 (fig. 15/), and the suppression of two of the remaining horizontal 

 rays a dermal rhabdus (fig. 15 g). The suppression of a distal ray, 

 excessive development of a proximal ray, and recurved growth of 

 the remaining rays produce an anchor. In Syalonema (glass rope 

 sponge) anchors over a foot long occur, but their arms or teeth are 

 not restricted to four, and the axial fibre gives off its processes 

 before reaching the head of the spicule. Such a grapnel helps to 

 support the sponge in the ooze of the sea-bed. Other character- 



FiG. 16. — a, unoinaria ; 6, clavula ; c, scopularia. After Sohnlze. 



istic spicules belonging to sponges distinguished by sexradiate 

 spicules are the following : — the undnarUt (fig. 16 a), a spinose 



oxea with the spines all pointing one way ; the davula, a tylotate 

 form with a toothed margin to the head (fig. 16 b) ; the scopulana 

 (fig. 16 c), a besom-shaped spicule with tylotate rays, which vary 

 in number from two to eight ; the amphidisk (fig. 15 d), a shaft 

 terminating at each end in a number of recurved rays. When the 

 sexradiate spicules of the ffexactinellida unite together in a manner 

 to be described later, the rays may be bent in a variety of waj-s 

 out of the triaxial type, so that the sexradiate character aloue 

 remains. 



Multiradiate Type.—Vue rays of an aster as of other spicules Multi 

 may be spined or tylotate. In one remarkable form known as a radia 

 sterraster (fig. 12 g, 'h), and characteristic of the family Geodinidai, type, 

 the rays are almost infinite in number, and coalesced for the greater 

 part of their length ; the distal ends, however, remain separate, 

 and, becoming slightly tylotate, are produced into four or five re- 

 curved spines, which give attachment to connective tissue fibres 

 by which adjacent sterrasters are united together. 



In one aberrant group of Lithistid sponges (Anomocladina) the 

 skeleton is formed of desmas, which are multiradiate^ each present- 

 ing a massive centrum (with an included cavity) produced into a 

 variable number (4 to 8) of rays, which rays terminate in expanded 

 ends (fig. 12/). 



It is doubtful whether a distinction between megascleres and Micro 

 microscleres can be maintained in the calcareous sponges, unless sclerei 

 the minute oxeas which occur in Eilhardia schulzei, Pol. {i6), are 

 to be referred to this gi'oup. They are widely distributed through- 

 out the sUicious sponges, and by their different forms afford charac- 

 ters of the highest importance in classification. 



One of the simplest forms is the sigmaspire (fig. 17 a,b); it looks 

 like the letter C or S, according to the direction in which it is 



^Al^ 



Fig. 17— Microscleres. a, i, sigmaspire viewed in different directions,— a, along 

 axis, and i, obliquely ; c, toxaspire ; d, spiraster ; e, sanidaster ; /, amphi- 

 aster ; g, sigma or cymba ; h, cymba, with three ptera at each end,— the central 

 one a proral pteron and the lateral, pleural ptera ; J, one end of another form 

 of cymba, showing seven ptera ; fc, monopteral cymba, — proral ptera only, 

 developed at ends, tropidial ptera much enlarged ; I, oocymba, in which proral 

 and pleural ptera have grown towards each other and coalesced ; m, spher- 

 aster ; n, oxyaster ; o, the same, with six actines ; p, the same, with four 

 actines ; g, tiie same, with two actines (a centrotylote microxea) ; r, micro- 

 tylote ; s, microxea (g, r, and s are reduced asters) ; t, rosette. 



viewed, its actual form being that of a single turn of a cylindrical 

 spiral. A turn and a part of a turn of a spiral of somewhat higher 

 pitch than that of a sigmaspire gives the toxaspire (fig. 17c); a con- 

 tinued spiral growth through several revolutions gives the poly- 

 spire. The sigmaspire becoming spined produces the spiraster or 

 spinispirula (fig. 17 d); this, by losing its curvature, becomes the 

 sanidaster (fig. 17 e), and by simultaneous concentration of its spines 

 into a whorl at each end, the amphiaster (fig. 17/). By reduction 

 of the spire the spiraster passes into the stellate or aster (fig. 17 n). 

 A thickening about the centre of the aster produces the spheraster 

 (fig. 17 m), allied to which is the sterraster. By a reduction in the 

 number of its rays the aster becomes a minute calthrops, from which, 

 by increased growth, the skeletal calthrops may very weU be derived ; 

 by further reduction to two rays a little rhabdus or microrabd re- 

 sults, and of this numerous varieties exist, of which the oxeate 

 microrabd is the most interesting, since it only differs in size from 

 the commonest of all skeletal spicules, the oxeate or acerate rhab- 

 dus. The sigmaspire is formed as a superficial spiral thickening 

 in the wall of a spicule cell or scleroblast j as superficial deposits 

 also the next group of spicules, the so-called anchorates, arise. 

 Take a hen's egg as the model of a scleroblast, draw round it a 

 broad meridional band, interrupted only on one side, for 30° above 

 and below the equator ; this will represent a truly C-shaped spicule, 

 which differs from a sigmaspire by the absence of spiral twist. 

 It may be termed a cymba (fig. 17 g). The back of the " C " is the 

 keel or tropis; the points are the prows orprorx. Now broaden out 

 the prora on the eggshell into oval lobes (proral pteres) ; and from 

 each pole draw a lobe midway between the prora and the tropis 

 (pleural pteres), and a common form of anchorate, the pterocyinba 



