Muscular Systems of the Horny Sponges, 373 



space, which is connected only with the afferent canal-system. 

 On the inner surface of the tubes and in the walls of the 

 lacunge no oscula are to be found. This vestibular structure 

 consequently differs considerably from that which I have 

 described in the case of the Aulenidse *. It resembles that 

 occurring in the Nardorus forms. 



The oscula are small and are always arranged in rows. 

 These rows traverse the finger-like processes in a longitudinal 

 direction. The finger-like processes are 10-20 millim. tliick, 

 and of about the same length. The oscula are 1-2 millim. 

 broad, circular, and placed at pretty regular distances of 10- 

 15 millim. apart. 



In the skeletons we see no trace of oscula. In place of the 

 rows of oscula there are in them deep furrows, which some- 

 times extend down into the central pseudo-oscular tube. In 

 the living sponge nothing can be seen of these furrows, wliich 

 are completely filled with sponge-tissue. It is true that this 

 tissue has no skeletal support, and in dried specimens this 

 part appears much depressed and as having fallen in. 



In transverse sections we see that the tissue which forms 

 this part is very loose and lacunar. The groups of flagellate 

 chambers between the broad and irregular, generally longi- 

 tudinally-directed canals are small and not very numerous, 

 in fact far less numerous than those of the much denser 

 skeletiferous part, which is furnished only with small canals. 

 These large canals and lacunar coalesce to form the short 

 oscular tube ; they all belong to the efferent system. 



While even in this furrow and its structure we have d, 

 remarkable peculiarity presented to us, we find much more 

 interesting characters on careful microscopic examination. 



The skeleton is a very finely reticulate Euspongia skeleton. 

 The main radial fibres are but little branched and bear sand. 

 The uniting fibres, on the contrary, are free from foreign 

 bodies and much ramified. They form numerous anasto- 

 moses. Their average thickness differs in the different 

 varieties, and varies between 001 and 0*02 millim. At the 

 margin of the furrow all these uniting fibres terminate in very 

 sharp ensiform points, wliich stand so close together that the 

 wall of the furrow appears densely spinose. There can be no 

 doubt that this spinosity is a defensive arrangement against 

 such foreign intruders as may attempt to penetrate into the 

 sponge-body from the wide oscular tubes and lacunas of the 

 efi"erent system. 



• Lendenfeld, " Ueber den Bau der Hornschwamme," Zool. Anzeiger, 



1885. 



