126 Prof. W. Thomson on the '■'■ Vitreous " Sjjonges. 



something in the wonderful complexity of design and profusion 

 of ornament in the siliceous skeleton which reminds one 

 strongly of the Polycystina ; and even, in some cases, the 

 special forms of the spicules are repeated in the two groups. 

 (Compare pi. 12. fig. 1, pi. 17. fig. 4, pi. 18. fig. 15, pi. 21. fig. 7, 

 pi. 32. figs. 10, 11, pi. 33. figs. 6, 7, &c. of Haeckel's ' Die 

 Kadiolarien'). As yet, we know nothing of any of the spe- 

 cies in a living state. The sarcode is certainly somewhat dif- 

 ferent in character from that of the other groups of Sponges, 

 — softer and more mobile, less loaded with granular formed 

 matter, and more transparent. For sarcode in this condition 

 we should be inclined to anticipate a somewhat higher form of 

 vital activity. It remains to be seen whether there may be 

 any approach by the extension of any form of pseudopodial 

 processes to the condition of the sarcode in the Rhizopoda. 

 Under a strong impression that it is through this order that 

 the Sponges pass into the Radiolaria, I have placed the POEI- 

 FEEA ViTKEA at the head of the series of siliceous Sponges, be- 

 ginning with those genera in which the siliceous elements are 

 most independent and varied, and the sarcode least consistent. 

 I believe that Dr. Gray has rightly indicated the base of the 

 order by placing the Flower-baskets next the Esperiadse, 

 which I should certainly regard as the inosculating family of 

 the Halichondrida. 



(Porifera Silicea). Order I. VITREA. 

 Habrodictyon, n. g. 



1. AJcyoncelhim, De Blainville, Quoy & Gaimard, 'Voyage de I'Astrolabe,' 



Zoologie^ vol. iv. p. 302. Paris, 1833. 



2. AlcyonceUiftn. Deshayes and Milne-Edwards, in Lamarck's ' Animaux 



sans Vertebres,' vol. ii. p. 589 (1836). 



3. Alcyonellum, Owen (misprint for Alcyoncellmn), Trans. Zool. Soc. vol. iii. 



p. 205 (1849). 



4. Alcyoncelliim, Bowerbank, Phil. Trans, and British Spongiadfe, vol. i. 



p. 174 (1865). 



5. EuplectcUa, Gray, Ann. & Mag. Nat. Hist. vol. xviii. p. 487 (1866). 



6. Corbitella, Gray, Proc. Zool. Soc. for 1867, p. 530. 



7. Heterotella, Gray, Proc. Zool. Soc. for 1867, p. 531. 



Sponge-body subcylindrical, tubular, attached by a slightly 

 contracted base. The walls of the tube composed of a per- 

 fectly irregular netAvork of bundles of siliceous needles 

 loosely and irregularly arranged in sheaves crossing one 

 another at low angles, and connected by a small quantity of 

 soft mucilaginous sarcode. The spicules of the skeleton all 

 essentially of the hexradiate form, free and separate from 

 one another, or rarely connected in groups of two or three. 

 The spicules of the sarcode very nimierous, " floricomo- 



