BY R. VON LENDENFELD, PH.D. 315 



form of Osculum may arise, by a net-work being formed which 

 closes the wide Osculum (Euplectella). Sometimes a thin chimney- 

 shaped membrane projects from the Osculum. (Caminus.) 



Sometimes a frill of large spicules surrounds the opening (many 

 Calcareous Sponges, Holtenia.) The tube itself may possess 

 transverse membranes, which by being more or less tightened 

 regulate the current of water (some Spongidse). 



5. The Skeleton. 



By far the greater portion of known Sponges possess a skeleton, 

 which in its shape and size is as variable as the shape of the 

 Sponge itself, but which in its minute structure appears rather 

 unvarying. The daughters may have a very different skeleton 

 from the mother as far as shape and size, probably also as far as 

 sti-ength is concerned ; but the minute structure will resemble that 

 of the parent so closely, that we are justified in looking on the 

 minute structure of the skeleton as a very important feature of 

 the Sponges in as much as their mutual relationship is concerned. 



Some Sponges — and as we may safely assume, those resembliug 

 the ancestral forms — have no skeleton. One or several of such 

 ancestral soft forms attained a skeleton, consisting of carbonate of 

 lime (Calcarea.) Others again produced fibres, which were 

 originally composed of fibrillous tissue only (GumminaB.) In the 

 course of time horny fibres were produced in the centre of the 

 fibrellous fibres, and these of course followed the direction of the 

 large canals (Oscar Schmidt). Originally they had a tree-shape 

 (Aphysillidae, von Lendenfeld, Vosmaer.) The branches coalesced, 

 and a reticulate structure was produced, in which main centrifugal 

 stems could be distinguished from tangental connecting fibres 

 (Spongidse, Aplysinida?. ) Foreign bodies, sand and the like, 

 adhering to the surface of the Sponge, in most cases entered the 

 horny fibres and filled t" e core of them in a varying degree. 

 Such foreign bodies ar mostly only to be found in the radial 

 fibres, but sometimes ? oin the tangental ones. 



