STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 207 



Syconoid to the Leuconoid condition. This change brings 

 about a loss of radial symmetry in the chamber skeleton, 

 which is transformed from the regular ^' articulate '^ type 

 of Sycetta^ Sycon, Grantia, to the irregularly scattered 

 condition of Leucaudra. Other modifications^ however, 

 also take place, which are obviously not dependent on 

 the variation of the canal system, and which are conse- 

 quently of the utmost importance for systematic purposes. 

 These modifications consist in the development of subdermal 

 sagittal triradiate or of subdermal quadriradiate spicules, 

 which characterise the two families Heteropidse and Ampho- 

 riscidse respectively. Both these modifications are first insti- 

 tuted (in the genera Grantessa and Heteropegma respec- 

 tively) while the canal system still retains its primitive 

 radial symmetry and its articulate tubar skeleton (figs. 18, 20), 

 and in both cases they are retained, while the canal system gra- 

 dually changes from the Syconoid to the Sylleibid or Leuconoid 

 type, and the primitive articulate tubar skeleton disappears 

 (figs. 19, 22). Thus the primitive centrifugal radial sym- 

 metry of the skeleton is lost as the canal system changes from 

 the Syconoid to the Leuconoid type, but in the Heteropidae 

 and Amphoriscidse it is to a certain extent replaced by a kind 

 of secondary centripetal radial symmetry, due to the develop- 

 ment of subdermal radiates with inwardly directed basal or 

 apical rays. Indications of the primitive radial symmetry are 

 sometimes found in the presence of subgastral sagittal trira- 

 diates after all other traces of the articulate tubar skeleton 

 have disappeared. The gastral cortex, as might be expected, 

 is subject to less modification than any other portion of the 

 skeleton, and does not vary to any great extent throughout 

 the group. A striking exception to this rule is, however, 

 found in the genus Utella. Very startling exceptions to 

 the ordinary rules of skeletal structure are found in the 

 genera Sycyssa and Leucyssa, apparently due to the loss 

 of all radiate spicules, and the development in their stead 

 of oxea. 



In Leucascus the skeleton appears never to have 



VOL. 35, PART 2. — NEW SEE. Q 



