E. MARSAALLI. — GEN. ARRANGEMENT OF SOFT PARTS. 125 



caiialar membrane (subgastral and subcanalar lacunœ) are never 

 specially widened as the subdermal lacunae or cavities are. 

 Hence, the layer occupied by them (i.e., the internal trabecular 

 layer) is much thinner and contains a much less quantity of the 

 trabeculse than does the peripheral trabecular layer outside the 

 choanosome. 



Haviijo" dealt with the ectosome and the endosome in their 

 relation to the skeletal parts supporting them, it may not be 

 amiss here to complete our account of the relation existing 

 between the soft parts and the spicules in the choanosome. 

 Leaving the hexasters out of question, the spicules and spicular 

 parts that enter into the composition of the choanosome are : 1) 

 the entire parenchymalia, 2) the proximal rays of the dermalia 

 and 3) the distal rays of the gastralia. All these are distributed 

 on either side of, and nearly completely separated into an outer 

 and an inner set by, the chamber-layer. At the oscular edge 

 the chamber-layer ceases to exist and the two sets of course 

 mix together. In other situations I hold it exceedingly doubt- 

 ful if there exist any spicules which penetrate right through the 

 chamber-layer. The point is rather difficult to settle by the 

 examination of sections and still more so by any other method, 

 but I have never once noticed, not only in Euplectella but also in 

 any other Hexactinellid that I have studied, a spicular ray which 

 undoubtedly passed through the chamber-layer. Certain it is 

 that the wall itself of the flagellated chambers is never pierced 

 through by spicules ; so that if ever a spicule does extend across 

 the layer, it must do so between the separate chambers. I 

 should think that the chamber-layer, in extending itself and 

 making evaginations with the growth of the sponge, pushes 

 its way in the intervals between the spicules present and that 



