REPORT ON THE HEXACTINELLIDA. i,0'i 



interjacent pores, then the triradiate spicule is the fittest form. These can be distributed 

 in such a way that each interspace between three adjacent pores is occupied by a regular 

 triact with each of the three rays lying at uniform angle between two adjacent ])ores 

 (fig. 10), or they may be disposed so that only half of the interspaces between the pores 

 are occupied by the central portions of the triacts, while the other half contain the points 

 of convergence of the extremities of three rays of three adjacent triacts (fig. 11). This 

 latter mode is exhibited by numerous very simple calcareous sponges of the Ascon type. 

 For the case of an open tube, fixed at one end, and with the other (oscular) extremity 

 free, careful consideration will show that the latter mode of disposition is the most 

 advantageous. In this way the boundary of each pore, especially on the lower mai-gin, is 

 strengthened by the forking of the triact which embraces it posteriorly, and the ^\'hole 

 sponge-tube is better strengthened by the relatively longer spicules than it would have 

 been on the former plan. We may therefore regard the development of the regular 

 triact as that conditioned and demanded by the structure of the soft parts of the 

 primitive calcareous sponges. 



In regard to the Tetraxouia, with their regular tetracts, 1 submit the folluwing con- 

 sideration. When a number of spheres of equal size are uniformly pressed together on 

 all sides, they become disposed to one another in such a way that between each four 

 adjacent and directly contiguous spheres a regularly formed cavity is left, which is 

 continued in four three-sided clefts disposed at a uniform angle, and is thus connected 

 with the adjacent interspaces of similar form. One can best compare the form of these 

 spaces to regular tetrahedra with inpushcd walls and drawn-out 

 angles, which pass into the similarly elongated angles of adjacent 

 tetrahedral spaces, and thus secure the connection of aU the 

 cavities. Now, if one supposes this entire system of cavities 

 to be filled with a semi-soM mass, and the spheres to be empty 

 spaces, there is an obvious necessity for a supporting framework. 

 And if the skeletal system necessary for the support of this 

 framework consists of uniformly movable skeletal elements with 

 cylindrical branches, then each of these bodies must necessarily '°' tacrwuirfnur'spher^ ™" 

 liave its centre in the middle of each tetrahedral mass between 



each four adjacent hollow spheres, and from this centre four strands must run out along 

 the four elongated angles of the tetrahedron. 



The best supporting element for such a mass is aflbrded by such regular tetracts as 

 we find in the similarly constituted parenchyma between the ciliated chambers of 

 Tetraxonia, and known to be typical for this group of sponges. 



Although the almost wholly unknown development of the HexactineUida gives us 

 as yet no basis for framing a conception of the architecture of the primitive Hexac- 

 tineUida, it is possible, from the close resemblance in the essentials of structure exhibited 



