BY R. VON LENDENFELD, PH.D. 505 



CANAL SYSTEM. 

 A. Vestibule spaces. 



On the summits of the digitate processes round holes are found, 

 which resemble Oscula very closely Theseare however the apertures 

 of the system of Lacunae, which must be considered as a vestibule 

 space, and have nothing to do with the exhalent system. The 

 vestibule cavity consists of wide canals with circular transverse 

 section, measuring from 5 to 12 mm. in diameter. These canals 

 are mostly upright, vertical and form here and there anastomoses, 

 about ^ to f of the volume of the sponge is taken up by these 

 vestibule spaces. 



They are connected with the outer world only by the Pseudoscula 

 on the summits of the processes. There are no other apertures, so 

 that the whole space appeal's as a perfectly closed Pseudogaster. No 

 Oscula open into these spaces, their walls are pervaded only by the 

 inhalent pores, and so these cavities appear as an outwai'd 

 appendage to the inhalent system ; they are perfectly homologous 

 to the vestibule spaces described above of Euspongia irregularis. 



B. True Canals. 



The canal system of our sponge is very remarkable. The pore 

 sieves are pervaded, as mentioned above by a great number of very 

 small pores. These are circular and can be entirely closed at the 

 option of the sponge. In consequence of this, the uvimber in each 

 pore sieve found open and visible is very variable. The greatest 

 number counted by me in one pore sieve was 20. They are circular 

 and scattered regularly over the whole of pore-sieve. They measure 

 when dilated O'Ol mm. in diameter. The pore sieve itself is a very 

 fine skin O'Ol mm, thick and attached to the sand-net which divides 

 the pore sieves from each other. 



These structures are perfectly similar in the outer, and inner 

 side of the lamellous body of the sponge. The pores of the 

 inner surface open into the vestibule, those of the outer surface 

 into the outer world. Below the pore-sieves extensive sub-dermal 



