14 B. W. TRIE8T ON THE HEXACTINELLID^E. 



1857, and the original sponge may now be seen and examined at 

 South Kensington. 



Very little is known, as far as I can find, of the life-history of 

 this order of sponges, the most, perhaps, being where Sir Wyville 

 Thompson, after describing Iloltenia Carpentei-i, says, " When the 

 sponge is living, the interstices of the silicious network are filled up, 

 both outside and in, with a delicate fenestrated membrane formed of 

 a glairy substance like white of egg, which is constantly moving, 

 extending or contracting the fenestra?, and gliding over the surface 

 of the spicules. This " sarcode," which is the living flesh of the 

 sponge, contains distributed through it an infinite number of 

 minute spicules, presenting the most singular and elegant forms 

 characteristic of the species. A constant current of water carried 

 along by the action of cilia passes in by apertures in the outer wall, 

 courses through the passages in the loose texture of the intermediate 

 sponge-substance, carrying organic matter in solution and rjarticles 

 of nourishment into all its interstices, and finally passing out by the 

 large osculum at the top." 



There is another sponge about which there has been much con- 

 troversy as to whether it should be placed among the Hexactinellidce 

 or the Tethyidce, viz., Dorvillia agariciformis of Kent, and Tethyea 

 muricata of Bowerbank. This sponge has some of the characters 

 of a Tethyea, but is certainly more closely allied to the order we 

 have been considering. It seems to be a link between the two, 

 showing how one species of sponge runs into another, as we may see 

 over and over again. 



A large number of the Fossil Sponges found in the chalk and 

 greensand, and known under the name of Ventriculites, belong 

 to the Hexactinellidce, and most of them show an octohedral 

 knot structure in the outer skeleton ; but Mr. Carter tells me 

 that he only knows of two existing species that show the same 

 structure, viz., Myliusia Grayi, a specimen of which is in the 

 Museum, and Myliusia Zittilii, from the Phillippine Islands, of 

 which he kindly sent me a fragment." 



DESCRIPTION OF PLATES. 



Plate II. 



Fig. 1, 2, 3, 4. — Typical spicules of the Hexradiate order of Sponges. 

 o & 7. — Anchoring spicules of llosse'la. 

 6. — Anchoring spicule of Pherunema. 



