The Sponges of Lake Biwa. 9 



tribution of the areas gives the surface of the sponge a very 

 characteristic appearance. 



There are no free microscleres. The skeleton-spicules are 

 short and rather stout, slightly curved, somewhat abruptly and not 

 very sharply pointed. Their axial tubule is often conspicuons. 

 Their external surface is often quite smooth, but sometimes the 

 middle region of the spicule bears a few scattered spines, and speci- 

 mens occur in which the majority — never all — of the macroscleres 

 are somewhat densely spiny in this region; the extremities are 

 always smooth. 



The gemmule-spicules are slender, cylindrical, feebly curved, 

 minutely and not very regularly or closely spined. They are either 

 sharply and gradually or abruptly and bluntly pointed at the 

 extremities. 



Gemmules are seldom produced. When they are found they 

 lie at the base of the sponge and adhere to the object to which it is 

 attached. They have not yet been observed in the free or encrusting 

 specimens. They are somewhat flattened in form and have a 

 rather thin granular coat and a short foraminal tubule, which is 

 situated on the upper surface in the natural position. 



As will be gathered from the above description, S. dementis 

 is of extreme variability not only in respect to the external form 

 and colouration but also as regards the structure of the skeleton 

 and the form of the spicule. Until we had examined a very large 

 series of specimens, in which forms intermediate in all respects 

 were found, we separated the specimens provisionally into four or 

 five distinct species or varieties. The comparatively stout skeleton- 

 spicules are a distinctive character so far as the Spongillidae known 

 from Japan are concerned, but in most specimens some spicules 

 can be discovered that are much more slender than the others. 

 When the majority of the spicules are spiny it is usually the 

 smooth ones that are the slenderest. It appears to be safe to say 

 that at least some of the skeleton-spicules are always at least -^ time 

 as broad as long. 



The difficulty of preparing an exact diagnosis of the species is 

 increased by the fact that gemmules are very seldom produced. 



