108 I. IJIMA : HEXACTINELLIDA. I. 



consequently become so concealed among the crowd of other 

 spicules more lately developed that they easily elude detection. 

 Further it may be that, as the sponge grows in length at the 

 superior end, the parenchymalia once occupying the edge may 

 be left behind, instead of being shifted along .and in perpetual 

 connection with the edge. 



As to the sieve-plate, in most specimens under oO mm. length, 

 I find it entirely or almost entirely lost, so that the gastral cavity 

 opens above by a wide circular aperture. The beams of the plate 

 in such small individuals are so thin, soft and excessively frail as 

 to break off on the slightest provocation. I have known them 

 to succumb to the rush of water as the freshly caught specimens 

 were being picked out of the sea. At other times I have seen 

 the air-bubble in the gastral cavity or the motion of alcohol 

 into which fresh specimens were thrown, disturb or destroy the 

 sieve- plate. In many cases, the soft and delicate beams were 

 severed clean off from the comparatively firm rim of the wall, 

 leaving no trace of the sieve-plate visible to the naked eye ; in 

 some other cases, they left behind as relics a greater or less 

 amount of shreds attached to the rim. In the specimens figured 

 in figs. 6-8, PI. IV, the sieve-plate Avas entirely gone. Only in 

 two specimens out of several measuring under 50 mm. in length, 

 do I find the plate nearly completely preserved by some fortunate 

 circumstance. I have given double-sized sketches of both these 

 cases in fig. lî> a & b. In the one specimen (39 mm. long), the 

 plate is scarcely or but slightly arched ; the meshes are ten in 

 number. In the other specimen (48 mm. long), it is convex, 

 like a watch-glass ; the number of meshes exceeds 10 b}^ a few. 



