4 3 



A longitudinal section of a representative of tropus tuberosa (92 a) finally, exhibits again 

 some main canals, surrounded with still more transparent tissue (PI. IV, fig. 3), but for the 

 rest resembling 612 a. 



We saw that in 1365 there is one very wide central canal, opening at the top of a long 

 process with a wide mouth. In other specimens of the same tropus and doubtless identical with 

 it, we find, however, more than one large central canal. Likewise, e. g. in 1975 (PI. IV, fig. 8). 

 This corresponds, consequently, whith what we found in the other tropi. A difference lies in 

 the fact, that both in 1975 and in 1365 the opening at the top is wide, whereas in the other 

 specimens it is narrow. We find, however, that this is of no fundamental signification. For in 

 1365, 1971, 1975 ar, d 2049 we find that some processes open at the top with a wide mouth, 

 others with a narrow one (PI. V, fig. 10); again others are closed. 



Carter, Dendy, Ridley & Dendy and others of course mentioned these wide canals, 

 which they consider to be exhalant and consequently they call the openings "oscula". It is 

 well known that we find such central canals in the majority of Sponges. Of several cases it 

 is demonstrated, that the said canal is the main excurrent canal or cloaca and the mouth the 

 so-called osculum. We ought not to forget, however, that in numerous other cases it is not 

 demonstrated, neither by serial sections nor by observation of the living animal. In the 

 case of 1365 I came, as stated above, to the conclusion that the wide central canal is an 

 incurrent one. From the above given statements may be deducted that those central canals 

 in the mentioned specimens are homologous formations and consequently likewise of an in- 

 current nature. 



In collaboration with my friend Pekelharing I have tried to give an explanation, or at 

 least a possible hypothesis for the origin of the water-current in Sponges. We demonstrated 

 (1908 a p. 15), that the water, leaving the mastichorions in some way or other arrivés in the 

 cloaca and escapes in the direction of minor resistence, which is in the cases we considered, 

 towards the "osculum". We then proceed: "thus in the central tube the water will obtain a 

 certain velocity, which, once established, acts as the flying-weel of an engine. Obviously the 

 absence of fiagella in the central tube is of great advantage. Every irregularity herc would 

 only diminish the effect of the flying-weel". Now we saw in Spirastrella purpurea that in 

 almost every case the mouth of the central canal is smaller than its general diameter. We 

 saw that in some cases this is due to contraction; but in others (305a, 305c!) the central canal 

 shows a ramification of narrower canals near the periphery, a phenomenon not to be explained 

 by contraction. Such contrivances, instead of producing in the central tube a locus minoris 

 resistentiae for the water current, would establish the contrary. As stated above, every irregularity 

 in the tube would diminish the effect of the flying-weel ; in 5". purpurea, we see rugae or 

 diaphragms instead of smooth walls. In the third place we find the large canals frequently 

 full of débris (Cf. supra and PI. V, fig. 2) ; a quantity of these are much too large to have 

 passed through the mastichorions. 



The above considerations gave me some doubt as to the excurrent nature of the large 

 central canals. It would be of high interest to observe these sponges in the living state. However, 

 these are pia vota and will probably long remain so. Consequently we have to try to make 



