374 Dr. R. von Lendenfeld on the Nervous and 



From the pointed ends of the horny fibres descends a mem- 

 brane which completely separates the lacmiar tissue of the 

 groove from the rest of the sponge-body. Below this mem- 

 brane is repeatedly interrupted, and here the efferent canals 

 come through. We find a membrane of this kind on each 

 side of the lacunar groove, and these membranes line the 

 side-walls of the groove throughout their whole length. 



On close examination of thin transverse sections it is seen 

 that this membrane is composed of parallel fusiform cells, 

 which stand perpendicularly to the outer surface of the sponge, 

 all of them descending radially towards its interior. They form 

 several layers in the above-mentioned membrane — generally 

 three. The membrane itself is of uniform thickness throughout. 

 These cells run out at both ends into extremely fine points. 

 They are 0"1 millim. in length and 0*003 millim. in breadth 

 at the middle. The oval nucleus is placed about the middle 

 of the length ; it is, however, not placed axially, but more or 

 less laterally. In the neighbourhood of the nucleus there is a 

 very small quantity of ordinary protoplasm, while all the rest 

 of the cell consists of a substance which differs essentially 

 from the contents of ordinary fusiform cells. Thus it con- 

 tains distinct, small, but strongly and doiibhj refractive, 

 rounded granules, imbedded in a homogeneous transparent 

 substance which is but slightly and simply refractive. The 

 granules are in part very regularly arranged, so that a sort of 

 transverse striation of the fibres is produced. The granules 

 do not combine to form doubly refractive disks, and the regu- 

 larity of their arrangement is not always equal in degree. In 

 examining material in spirit one easily sees that these mem- 

 branes of the walls of the groove are strongly contractile, and, 

 indeed, that they always contract in a radial direction. By 

 this means the outer surface of the tissue occupying the groove 

 is more or less lowered ; and in the very considerable 

 variation of depth to which the surface of the tissue occupying 

 the groove sinks in difterent specimens, we have the expres- 

 sion of the action of the radial contractions of these mem- 

 branes. 



I think we may conclude from the above-cited observations 

 that these membranes are muscles and the cells composing 

 them muscle cells ; and, further, that these muscle-cells in 

 their peculiar structure make the transition from the smooth 

 to the transversely-striated fibres. 



In transverse sections through the margins of the groove 

 we see that a peculiar organ is seated upon the upper outer 

 margin of this muscular membrane. We then find the mem- 

 brane suddenly increased to twice or three times its diameter 



