( 1141 ) 
How is it now with the internal structure ? The answer to this 
question is that we find here similarly great differences, but likewise 
all gradually passing into each other. This is true as well with re- 
gard to the canal system, as to the structure of the parenchyma and 
the skeleton, even with respect to the elements of the latter, viz. 
the spicules. I do not wish to discuss now these points in extenso ; 
I will give a couple of examples, as I did regarding the external 
habitus. 
It is characteristic for Spirastrella that the skeleton consists of 
longitudinal bundles of spicules, chiefly tylostyles, branching towards 
the periphery; between these bundles, but more especially forming 
a superficial crust, we find the well known spinispirae. It is a remarkable 
fact, that this represents the complete skeleton; under certain 
circumstances, however, the spinispirae may become very scanty and 
may even disappear altogether. In the latter case the character from 
which the genus has received its name, is absent *). In the papers 
of almost every spongiologist we find that species are distinguished 
e. g. on account of the size of the spicules. In our case the size of 
the tylostyli shows considerable differences. The maximal length 
I found in my specimens to oscillate between 280 and 800 u., whereas 
the maximal diameter varies between 5 and 30 u. Nevertheless, the 
extremes are united by all possible intermediate stages; no limits 
can be drawn for specifie distinction. There are, however, certain 
sizes which prevail. The maximal length is in about 33 °/, cases 
550 u and more, but less than 600 u; in about 75°/, cases 500 u 
and more, but less than 650 u. Cases of less than 350 u or more 
than 700 wu are rare. And so I found in 20 °/, cases the maximal 
diameter of the tylostyli 16 u and more, but less than 22 u. I seldom 
found it less than 6 u or more than 24 u. 
Stull more striking are the differences of the spirispirae. We have 
already seen that these characteristic spicules sometimes occur in great 
abundance, at another time are exceedingly scanty. Thus I found in 
some specimens that one microscopical preparation of spicules, isolated 
by means of hydrochloric or nitric acid, contained a couple of spini- 
spirae, whereas another slide of the very same specimen contained 
not a single one. Dexpy found in his Suberites inconstans var. globosa 
and var. maeandrina no spinispirae. | was able to examine DeNDY’s 
types in the British Museum and did find them. Such facts are by 
1) This phenomenon is by no means seldom in the animal kingdom. It may 
perhaps be compared with the absence of a chorda in certain Chordata. The more 
so as I have reason to believe that the said spicules are present in larval or 
young stages, but afterwards disappear. 
