90 MANUAL OF ZOOLOGY. 



persistence of their axial canals (fig. 28, C). The " flesh-spicules " are 

 fundamentally six-armed, but give off secondary branches so as to form a 

 rosette. 



Among the living Hexactinellidce, the Venus' Flower-basket (Euplectella) 

 is one of the most familiar forms. In this exquisitely beautiful sponge, the 

 skeleton-spicules are of large size, and the entire skeleton is at first flexible 

 and soft, the spicules being free. Ultimately, the spicules become cemented 

 together by a coating of vitreous silex, so as to form a ladder-like trellis- 

 work. There is a single terminal osculum, provided with a porous lid ; 

 the sponge-body is rooted in the mud of the sea-bottom by a beard of long 

 siliceous fibres ; and the entire skeleton in the living state is completely 

 concealed by a thick covering of brown sarcode. Another very interesting 

 Hexactinellid sponge is the Hyalonema or " Glass-rope Zoophyte," long 

 supposed to be a kind of coral. In this singular type, there is a com- 

 paratively small sponge-body, which is rooted to the mud of the sea- 

 bottom by a long rope of delicate siliceous fibres. In addition to this skein 

 of "anchoring-fibres," there are branched spicules, which are four-armed 

 or five-armed in the recent forms, but are hexradiate in fossil examples. 

 Other well-known living Hexactinellidcz are Aphrocallistes, Farrea, Dactyl- 

 ocalyx (fig. 28, A), &c. All the known forms are marine, and are inhabi- 

 tants of deep water. 



2. LiTHiSTiDy'E. These are siliceous sponges in which the spicules 

 are essentially quadriradiate, three of the four arms being so disposed as 

 to come together at an angle of 120, while the fourth arm lies in a different 

 plane to the others, and forms a cylindrical 

 shaft from which the latter spring. The ex- 

 tremities of the arms of the spicules are divided 

 into processes (fig. 28, B), and by the interlock- 

 ing of these, contiguous spicules are united into 

 a continuous skeleton, the meshes of which are 

 more or less irregular and curvilinear. 



Like the Hexactinellids, the Lithisiida are 



-..,-. ^~_^ all marine, and inhabitants of deep water; 



1 | Discodermia, Corallistes, M'Andrewia, Azorica, 



s^jUJii^. and Leiodermatium being well - known recent 

 ^ ^ x^^T^fai^ genera. 



The reproduction of sponges may be 

 effected either asexually or sexually, the 

 following being a brief outline of the 

 phenomena which have been observed 



F} tsto* m ^ e common fresh- water sponge (Spon- 



:9 ' AHihlm; \ Diagram- gillo), in which the process was first accu- 



matic section of the gem- j-ofgl v 

 mule, showing the outer / 



layer of amphidiscs and the 



inner mass of cells ; c One In the first or asexual method of reproduction, 

 of the amphidiscs seen in which takes place in the winter, the deeper por- 

 P rofile - tions of the sponge are found to be filled with 



small seed-like rounded bodies, termed "gem- 

 mules "or " spores," each of which possesses a small aperture or " hilum " 

 at one point (fig. 29, h\ Each gemmule is composed of an outer coriace- 

 ous capsule surrounded by a layer of peculiar asteroid spicula, resembling 

 two toothed wheels united by an axle, and termed "amphidiscs" (fig. 

 29, 3, c}. These amphidiscs are embedded in sarcode, whilst their inner 

 surfaces rest upon the tesselated capsule already mentioned. In the inte- 



