ADDENDUM TO SPONGES 



167 



about 10 mm. in height by 5 mm. in brcatlth. The openin<;8 of the 

 canal system are confined to the upper surface (Fig. 97, A; cf. Ttntwiwin^ 

 Fig. 31). 



Tlie skeleton of A.'^trosclera is composed, not of spicules, wliich are 

 entirely wanting, but of calcareous spherules, which arise in cells of tlie 

 dermal layer near the upper surface. Each spherule is deposited witliin 

 a single cell, and is from tlie first composed of radially arranged crystalline 

 fibres. Its form is at first spherical, but by further increase in size 

 adjacent spherules come into contact, and the interspaces between them 

 become comjiletely filled in by continued deposition of the calcareous 



/'^--/i^xJm 



Via. it". 



Astrosil''r<i WiUiijium, Lister. A, tlie sponge magiiilied about three diameters; p.s., upper 

 surface earrj'ing tlie openings of the canal system ; h, base of attachment. B, section of tlie 

 skeleton ; s/A., spherules ; c, canals. (Drawin.i,'s by Mr. J. J. Lister.) 



substance, to the exclusion finally of the soft parts. The .spherules thus 

 acquire a polyhedral form (Fig. U7, B, s'ph.), and by their union build up 

 a solid calcareous skeleton without any admixture of soft parts, but 

 traversed by canals in which are lodged the soft tissues and the canal 

 system of the sponge (Fig. 97, B, c). In the basal (older) jiart of the 

 sponge the canals became obliterated, apparently by extension inwards of 

 the spherules forming their wall ; just as in pedunculate sponges the 

 canal system is wanting in the stalk. 



The spherules are composed of aragonite, and contain an organic 

 basis which has the sam.e staining reactions as that of the spicules of 

 Calcarea. 



The canal system is of a leuconoid type with small sj)herical ciliated 

 chambers opening into branched canals. There is no large central 

 gastral cavity, but a number of excurrent canals [wliich perhaps represent 

 gastral cavities reduced in size] run vertically upwards to open on the 

 upper surface, alternating with incurrent canals similar in appearance and 

 arrangement. Some ova and larvae were observed, the latter apparently 

 of a parenchymula type. 



The affinities of this curious organism are very doubtful. It is un- 

 questionably a sponge, and the material of its skeleton is carbonate of 



