54 



formation of a continuous and firm skeleton-net ; nevertheless one often finds 

 adjacent spicules connected by synapticula so as to form ladder-like structures. 

 In respect to the developement of these secondary skeletal structures EvpledelJa 

 simplex stands on a level with Euplectella oweni. 



The most conspicuous spicules of the supporting skeleton are the stout and 

 long stauractines. In older specimens these measure 8 cm. and more in length 

 and 2 cm. and more in breadth. The thickness of their rays generally depends 

 on the age of the sponge. The thickest measure basally 100 /^ in diameter. 

 The centres of the stauractines usually lie in the nodes of the skeleton-net. The 

 longitudinal spicular fibres are nearly straight, the transverse fibres ring- or 

 hoop-shaped. The longitudinal rays of the stauractines lie in the former and 

 are also straight accordingly ; their lateral rays lie in the latter and are accord- 

 ingly also circularly curved and concave towards the axis of the sponge. Thus 

 the rays of the stauractines do not lie in a plane but in a cylinder-surface ; they 

 are smooth, round, and gradually attenuated towards the sharp-pointed end. 



To each of these principal spicules 10-30 much thinner and very long accom- 

 panying spicules — comitalia — are attached. The longest rays of the comitalia 

 are situated longitudinally and transversely, mostly parallel to the rays of the 

 stout principalia. Most of the comitalia are triactines, diactines are however 

 also numerous; occasionally pentactines or even hexactines are met with. In the 

 spiral spicular fibres the comitalia are not so numerous as in the longitudinal and 

 transverse ones. Those rays of the triactine to hexactine comitalia, which are 

 not parallel to the rays of the principalia are much shorter than the others and 

 not unfrequently curved. Sometimes they reach another parallel fibre and then, 

 bending round abruptly, follow it ; sometimes they extend radially through the 

 parenchyme, either towards the outer or towards the inner surface of the tube- 

 wall and form a support for the soft parts overlying the inner and the outer side 

 of the main skeleton -net. These radial comitalia-rays are generally conically 

 pointed and slightly inflated and roughened just below their end. 



In the irregular skeleton-net of the terminal sieve-plate no differentiation 

 between stout principalia and slender comitalia is observed and the large staurac- 

 tines are absent. The strands of the skeleton-net in the sieve-plate are composed 

 nearly exclusively of triactines and diactines with long rays. The latter usually 

 have a central thickening. Besides these spicules also numerous oxyhexactines 

 of the same kind as those forming the support of the parenchyme overlying the 

 skeleton-net, are met with. 



The soft parts on the inner and outer side of the main skeleton-net are 

 supported by the gastral and dermal comitalia-rays which do not take part in the 

 formation of fibres; by the radial rays of the hypodermalia and hypogastralia ; 

 and by numerous parenchymal oxyhexactines four rays of which are parallel and 

 two rays of which are vertical to the inner and outer surfaces of the tube-wall. 



