200 



PORIFERA 



the same layer, added to the more obvious character of thimble- 

 shaped chambers, are the chief archaic features of Hexactinellid 

 morphology. 



The skeleton which supports the soft parts is, like them, 

 simple and constant in its main features. It is secreted by 



scleroblasts, which lie in the 

 trabeculae, and is made up of 

 only one kind of spicule and its 

 modifications. This is the hexac- 

 tine, a spicule which possesses six 

 rays disposed along three rect- 

 angular axes. Each ray contains 

 an axial thread, which meets its 

 fellow at the centre of the spicule, 

 where they together form the 

 axial cross. Modifications of the 

 hexactine arise either by reduc- 

 tion or branching, by spinulation 

 or expansion of one or more of 

 the rays. The forms of spicule 

 arising by reduction are termed pentactines, tetractines, and so 

 on, according to the number of the remaining rays. Those rays 

 which are suppressed leave the proximal portion of their axial 

 thread as a remnant marking their former position (Fig. 94). 

 Octactine spicules seem to form an exception to the above state- 

 ments, but Schulze has shown that they too are but modifications 

 of the hexactine arising by (1) branching of the rays of a 



P'iG. 91. — Portion of a section of the 

 menibraua reticularis or chamber- 

 wall of Schaudinnia arctica. x 1500. 

 (After F. E. Schulze.) 



Fig 



92. — A, discohexaster, 

 la which tlie four cladi 

 a, a', b, b', c of each ray 

 start directly from a cen- 

 tral nodule. B, disco- 

 octaster, resulting from 

 the redistribution of the 

 twenty -four cladi of A 

 into eight groujis of 

 three. (After Schulze, 

 from Delage. ) 



hexactine, followed by (2) recombination o'i the secondary rays 

 (Fig. 92). 



The various spicules are named, irrespective of their form, 

 according to their position and corresponding function. The 



