AFFINITIES OF THE GENUS SIPHONIA. 805 



form along its vertical axis produces a series of oblate ellipsoids, S. 

 incrassato, Goldf. ; and again, if it enlarges at the base, conical forms 

 result, 8. conica, Court., Jims, Goldf., Fittoni, Mich., piriformis, 

 Sow. ; or if at the summit the forms become obconic, S. piriformis, 

 Goldf., Mich., Court. ; several heads of this form borne on a branch- 

 ing raceme constitute S. arhuscula, Mich. Finally, by unequal 

 lateral growth, lobations more or less numerous and pronounced arise, 

 and we have the different varieties of S. costata. 



At the apex of the body is the opening of a central canal, which 

 descends for a variable depth towards the base of the sponge, some- 

 times nearly reaching the stem, at others forming only a shallow 

 saucer-shaped depression ; while occasionally, as in S. Websteri, it 

 may be altogether absent. In breadth it is equally variable : in some 

 cases a large sponge is perforated by a long but very narrow canal ; 

 in others a much smaller specimen is widely excavated by a broad 

 funnel-shaped cavity ; and if S. eapansa be, as I believe, a member 

 of the genus, the widening and deepening may become so great as 

 to convert the canal into a large cup-like hollow. 



In a unique specimen lent me by Mr. Wm. Yicary, of Exeter, the 

 sponge is incompletely divided by longitudinal fission into two parts, 

 each of which possesses its apical opening and central canal. 



The sides of the central canal or axial tube are perforated by a 

 number of round or oval openings about ^ to -^L- inch in diameter, 

 situated at about equal distances from each other, but not exhibiting 

 either a quaternary or quincuncial arrangement; they manifest, 

 however, a tendency to succeed one another in tiers of circular rows. 



These openings are seen in vertical sections to be the distal ter- 

 minations of a number of canals which diverge from the axial tube 

 and perforate the sponge ; those which open into the basal part of 

 the tube continue its axial direction downwards, and are prolonged 

 into the interior of the stem below, while those which open into the 

 sides spread out in descending curves. As the canals open nearer 

 the summit of the central tube, so they lie nearer the exterior of the 

 sponge ; and as they approach the exterior, so they become more 

 parallel with its contour. 



In many specimens the most exterior of these canals are freely 

 exposed on the surface, radiating away from the edge of the summit, 

 opening as winding, bifurcating, and occasionally anastomosing fur- 

 rows. From this we might infer that the more internal canals, 

 which are not so clearly revealed, possess the same characters ; and 

 a tangential section made through one of the phosphatic specimens 

 from Folkestone proves that such is really the case, the windings, 

 branchings, and anastomosis being all well displayed. 



We have now described the " excurrent " system of the sponge. 

 The axial tube is the " cloaca ; " and the openings in its walls arc the 

 " oscules " of the longitudinal or excurrent canals. 



The exterior surface of the sponge -body is pitted by a number of 

 irregularly dispersed more or less circular holes about -^ inch diam., 

 which are the openings of canals which radiate inwards, normal to 

 the surface, towards the cloaca of the sponge, crossing the excurrent 



Q.J.G.S. No. 132. 3g 



