20 PLATE IX. 



POLYMASTIA ORNATA, BowerltauTc. 



Vol. ii, p. 58, ' Mon. Brit. SpongiacUe.' 



Fig. 13. P. ornata. The type-specimen natural 

 size. A portion of the proximal end of the sponge has 

 been torn off, exhibiting an interior view of the primary 

 fasciculi of the skeleton and their spiral mode of 

 arrangement. 



Fig. 1-4. One of the large acerate spicula of the 

 primary lines of the skeleton. X 23 linear. 



Fig. 15. One of the short, stout, acerate, external, 

 defensive spicula. X 123 linear. 



Fig. 16. A portion of the distal termination of a 

 specimen of P. ornata, sent to me by the late Mr. 

 Barlee. X 36 linear. This specimen exhibits the 

 excurrent orifices of the cloaca of the sponge under 

 consideration in a very satisfactory manner, and it also 

 forms an excellent illustration of the general form and 

 arrangement of the tissues in the excurrent organs of 

 this genus of sponges. 



The terminal excurrent orifices of the fistulce in 

 Polymastia are precisely similar to the corresponding 

 organs in Alcyoncellum, but they differ in the nature 

 of the structure of the reticulation surrounding each 

 of them. In the latter genus it is always composed of 

 more or less solid siliceous fibre ; in the former it is 

 always formed of interlacing fasciculi of spicula. The 

 design of the skeleton structure is the same in both 

 genera, but the modes in which it is carried out are 

 different, and the same modifications of the structural 

 materials exist in all parts of the skeleton structures 

 of the various species of the two genera. 



There is also another distinctive character of great 

 value in the separation of the two genera, and that is 

 the total absence in Polymastia of the numerous, large, 

 stout, rectangulated, hexradiate, interstitial spicula that 

 are so abundant in Alcyoncellum around every one of 

 the inhalent areas of that sponge. 



