MICROSCOPICAL STUDIES. 41 



dense stony defensive layer. Other minute spicules (4 e — 4 h) are 

 found scattered through the tissues. As to the larger spicules, the 

 megascleres, the principal or four-axial ones peculiar to the order, are 

 here disposed as a supporting scaffolding beneath the cortex. The 

 form they take (Fig. 4 6) is known as the orthotrisene. Others of 

 simple structure (4 a) are located in the inner tissues. 



The water of circulation enters through sieve plates into large 

 subdermal chambers, partitioned into two by a strong sphincter 

 muscle, able thus to close the opening on necessity. From the 

 inner chamber proceed the incurrent canals feeding the ciliated 

 chambers. 



Tethya lyncurium (Johnston) is in many respects a remark- 

 able sponge. In appearance, it simulates perfectly a miniature 

 tangerine orange in colour and in shape. Extremely spicular and 

 furnished with a cortex — a true rind — as well developed as is that 

 of Pachymatisma, no pores or oscula can be made out by the naked 

 eye when the sponge is out of water, but in life in its native element, 

 the oscula seem to be collected upon a slightly papillate prominence 

 near the apex. Large canals are very sparse and it is at first difficult 

 to believe this sponge to possess a complicated canal system as it 

 undoubtedly does. The megascleres are strangely not four rayed 

 as we should expect, but are long slender rods (3 c — d) gathered 

 into great radiated bundles, quite obvious to the naked eye in a 

 hand section (3 b). Microscleres, in the form of beautiful starry 

 bodies, abound (3 / & 3 g). As regards ciliated chambers, I have to 

 confess I have never seen any in either Pachymatisma or Tethya. 

 But this is not to be wondered at perhaps, as the size of even the 

 largest ciliated cells of any siliceous sponge is very small compared 

 with that of those of the calcareous species. 



Explanation of Plate III, Fig. 1-4 & Pl. IV, Figs. 1-8. 



Sponge anatomy, &c. 

 Plate III, Fig. 1. Halichondria panicea, x |; a. megasclere. 

 Fig. 2. Clathria seriata, x § ; a, megasclere ; b, micosclere (toxon). 



Fig. 3. Tethya lyncurium ; a, another (smoother) specimen ; b, 

 same split open to show cortex and radiating spicule 

 bundles ; all x § ; c — e, megascleres ; / & g, mi- 

 croscleres. 



Fig. 4. Pachymatisma johnstonia X f ; a & 6, megascleres ; 

 c & d, sterrasters ; f & h, other microscleres. 



