BY R. VON LEXDENFELD, PH.D. 979 



and more tangental direction. In most parts of the Sponge these 

 processes at the same time diminish in number till we finally 

 arrive at structures which no longer can be termed star-shaped, 

 but which are already decidedly bipolar, spindle-shaped. Such 

 bipolar tissue cells are very frequent in other Sponges. In the 

 Calcispongia? they are. comparatively rare, and met with only in 

 the Heterocoelia. They are here perhaps to be considered as 

 muscular element throughout. It is in the Calcispongiae still more 

 difficult than in others to discern between contractile cells and 

 ordinary tissue elements. 



In the Homocoela the movements (closing of Pores) are 

 doubtlessly caused by contractions of the Processes extending 

 from the star-shaped tissue cells. 



SKELETON. 



Spicules, with two, three and four axis, comparable to the axis 

 of crystals, are always found in great abundance in the Calci- 

 spongise. They consist of carbonate of lime, mixed with organic 

 substance. They often protrude beyond the Mesoderm. But 

 they are, as mentioned above, always covered by a mesodermal 

 cuticule. The structure of the spicules can best be studied in 

 specimens treated with chloride of gold-potassium. (Twelve 

 hours.) By means of the appliance of this re-agent we see that 

 the whole spicule consists of a great number of small prisms, 

 parallel to one another, radiating from the axis. Under ordinary 

 circumstances no such structure can be detected, but by the 

 appliance of the gold solution we can readily isolate these prisms. 

 A spicule exposed to this re-agent represents melting glacier ice in 

 appearance. Like the latter it is composed of parallel prisms. 



Furthermore we find that the radial structure first makes its 

 appearance in the interior, close to the inner axis, which is a 

 cylindrical chord of organic matter without lime. The whole 

 spicule soon commences to be dissolved by the gold solution if the 

 latter is strong, and always from the axis outward. The outermost 

 layer remaining intact whilst the central parts are split up into 

 prisms and dissolved. These new observations, together with 

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