34 



Dr. Wyville Thomson on Holtenia. [June 17, 



Smaller stars, formed by the radiation of smaller spicules of the same 

 class, occupy the spaces between the rays of the larger stars. 



The rays of each star bend irregularly, and meet the rays of the spi- 

 cules forming the neighbouring stars. The rays of the different spicules 

 thus run along for some distance parallel to one another, and are held 

 together by a layer of elastic sarcode, which invests all the spicules and 

 all their branches. Between the rays of the spicules, over the whole sur- 

 face, the sarcode forms an ultimate and very delicate network, its meshes 

 defining minute inhalent pores. 



At the top of the sponge there is a large osculum, about 3" in diameter, 

 which terminates a cylindrical cavity, which passes down vertically into 

 the substance of the sponge to a depth of 5" 5'". The walls of this 

 oscular cavity are formed upon the same plan as the external wall of the 

 sponge ; and the stars, which are even more conspicuous than those of the 

 outer wall, are due to the same arrangement of spicules of the same form. 

 The ultimate sarcode network is absent between the rays of the stars of 

 the oscular surface. 



The sponge-substance, which is about 2" in thickness between the 

 oscular and the outer walls, is formed of a loose vacuolated arrangement 

 of bands and rods of greyish consistent sarcode, containing minute dis- 

 seminated granules and groups of granules of horny matter, and minute 

 endoplasts. 



Towards the outer wall of the sponge the sarcode trabeculse are arranged 

 more symmetrically, and at length they resolve themselves into distinct 

 columns, which abut against and support the centres of the stars, leaving 

 wide, open, anastomosing channels between them. The sarcode of the 

 outer wall, and that of the wall of the oscular cavity, is loaded with minute 

 spicules of two principal forms, quinqueradiate spicules with one ray 

 prolonged and feathered, and minute amphidisci. 



Over the lower third of the body of the sponge, fascicles of enormously 

 long delicate siliceous spicules pass out from the sarcode columns of the 

 sponge-body in which they originate, through the outer wall, to be diffused 

 to a distance of not less than half a metre in the mud in which the sponge 

 lives buried ; and round the osculum and over the upper third of the 

 sponge, sheaves of shorter more rigid spicules project, forming a kind of 

 fringe. 



The author referred all the sponges which were found inhabiting the 

 chalk-mud to the Order Porifera Vitrea, which he had defined in the 

 'Annals and Magazine of Natural History' for February 1868. This 

 order is mainly characterized by the great variety and complexity of form 

 of the spicules, which may apparently, with scarcely an exception, be re- 

 ferred to the hexradiate stellate type, a form of spicule which does not 

 appear to occur in any other order of sponges. The genus Holtenia is 

 nearly allied to Hyalonema y and seems to resemble it in its mode of occur- 

 rence. Both genera live imbedded in the soft upper layer of the chalk- 



