Phyhgeny of the Ampliidiscopliora. 481 



delicate cuji-sliapc'l syncytiul neUs'ork, with the ga3(ral layer 

 suspended midway between outer and inner layers of dermal 

 reticulum ; large hexactin.s or their derivatives form the main 

 scaffolding, and small ones support and keep open tiie delicate 

 meshes of the network. 



To understand the origin of the hexasters with turned- 

 down end rays (hexadisks) and of the hexasters with 

 turned-up end rays (hexasters proper) we must seek for the 

 cause in the structure of the sponge reticulum. Amphidisks 

 (j. e. reduced hexadisks) are mainly distributed about the 

 bounding surfaces (fig. 1), which usually have a papyraceous 

 texture. Amphidisks are adapted for keeping apart parallel 

 membranous laminte of sponge reticulum ; and hexasters, for 

 maintaining in an expanded state cubical areas of delicate 

 sponge reticulum by prodding out the strands of the network 

 by means of their end rays. 



Obviously the hexadisks would soon become reduced to 

 diactin spicules, for the four tangential rays would be acting 

 in the void. Hexadisks still persist in Monorhaphis dives, 

 F. E. Sell. (fig. 2), and now and then as s})orts in other species 

 — a reversion to an earlier and less efficient condition. 



The amphidisk teeth, with all their wonderful develop- 

 ments, have arisen in response to the necessity for keeping 

 the spicule orientated at right angles to the opposing planes 

 of tissue, and of restoring it to its position when displaced. 

 They serve, in fact, as the points d^appui for bands of con- 

 tractile tissue passing from the parallel planes to the teeth. 

 A simple wooden model of an amphidisk placed between two 

 sheets of elastic, and witb threads of elastic passing from 

 holes in the periphery of the disks to the elastic sheets, will 

 demonstrate the utility of the balancing fibres, for without 

 them it is diflficult to keep the spicule vertical or to restore it 

 when out of place *. 



Some abnormal or exceptional forms of amphidisk are 



* Dwellers in earthquake countries miplit, perhaps, with advantag^e 

 fullow the methods adopted by the Aniphidiscophoran sponge, for in each 

 case there is the same problem to solve. The amphidisk has to keep 

 apart laminje, to be vertically orientated, and to be readily restored to its 

 position when displaced. Similarly, in cases where the earth-pellicle is 

 unstable, the house has to be kept upright, and floor and roof kept apart. 

 The inhabitants, then, should become " Ainpbidiscoeci " or dwellers in 

 amphidisks. The design would consist of a gigantic amphidisk in a 

 cubical scaffolding, witli jointed iron staples passing from the teeth of 

 the disks to the upper and lower planes of the scatfokiing ; there would 

 be a domed roof, and the polar area of the lower disk would tit into a 

 •wide socket. The walls could be made of some strong light material, 

 such a,s papier-mach^, in larged framed plaques jointed together. 



Ann. (fc Mag. X. Hist. 8er. 8. Vol. iv. 34 



