REPORT ON THE HEXACTINELLIDA. 501 



derived allies the Mouaxonia and horny sponges the regular four-rayed form (spanischc 

 Reiter), and in the Triaxonia (Hexactiuellida) the regular six-rayed spicule, it seems to 

 me of essential importance to note the diflerencc of tlic architecture of the soft paits in 

 these three principal groups. 



The Ascones, which may be regarded as the starting group in the calcareous sponge 

 series, exhibit, as is well known, in the simplest instance, a fixed thin-walled tube open at 

 the free end, and with its side wall penetrated by uniformly distributed circular pores. 



In the much-varied group of Tetraxonia and their descendants we may consider the 

 typical form as that of a thick-walled cup, in the compact wall of which roundish or 

 quite spherical ciliated chambers lie crowded together like the acini in an acinose gland. 

 Such at least are most of the Tetractinellida and Lithistida, numerous Monactinellida and 

 horny sponges, though deviations also occur, as in the flat crusts of many Plakinidae, 

 which can hardly be regarded, however, as typical or primitive forms. 



The typical structure of the Hexactinellida is very diff"erent. 



The exceedingly loose wall of the typically .sack-shaped body exhibits, between two 

 abundantly penetrated, thin, parallel bounding lamellae, a single layer of large, sack-shaped, 

 ciliated chambers, connected both with the outer dermal and inner gastral membrane 

 by a trabecular system of thin thread-like strands. In simple and young forms these 

 thin connective-tissue trabeculae run predominantly in a radial direction between the 

 cliamber-layer and the two limiting lamellse, or directly between the latter, and are 

 usually so closely united with one another by tangential uniting strands that one 

 usually observes six threads at right angles to one another at each node of intersection. 

 This simplest structure is not of course persistent throughout. And in many cases, 

 doubtless, the insufficient preservation of the soft parts does not permit of the recogni- 

 tion of the original disposition of the trabecular. 



Let us now consider how the spicules, regarded as typical and primitively charac- 

 teristic, are disposed in the A^ry variously con.stituted tissue of these three divisions of 

 sponges. 



The regular three-rayed spicules of the Ascones are well known to occur tangentially 

 embedded in the wall of the tube, and in such a way that one ray lies parallel to the 

 axis dii'ected backwards towards the base, while the two others run ol)liquely forwards 

 and to the side, each usually embracing a parietal pore posteriorly. 



In the Tetraxonia the typical regular tetracts lie in their simplest and most normal 

 form between the spherical ciliated chambers, while, as a rule, the regions without 

 chambers, viz., the margin, basis, and neighbourhood of the large canals, contain more 

 or less markedly differentiated spicules. 



In the Triaxonia (Hexactinellida), finally, the typical regular hexacts arc found 

 almost exclusively in the strands of the trabecular framework, while in the chamber- wall 



(ZOOL. CHALL. EXP. — PART LIII. — 1887.) Ggg 64 



