12 THE VOYAGE OF H.M.S. CHALLENGER. 



And further : " Supposing that these invaginations of the gastric wall take place 

 near one another, not so regularly, and that on their walls secondary invaginations arise 

 in their turn ; supposing that along with this a stronger growth of the connective tissue 

 takes place, it becomes obvious that the ectoderm and endoderm cannot continue to 

 progress side by side, in other words, that both the layers of epithelium can no more, as 

 in the Ascon and partly in the Sycon , run parallel to each other. We have seen that 

 in the Sycon the flagellated epithelium is confined to the radial tubes alone ; if now in 

 the case just mentioned it recede still more, we have the picture of the exhalent canal 

 system of a Leucon. The ectoderm, which at first lay close upon the endoderm, is often 

 displaced from the latter by the connective tissue ; it cannot follow the invaginations of 

 the endoderm throughout. And yet both the layers are, so to speak, attracted one to 

 another, and where there is a less quantity of the mesoderm, the original pores break 

 through. One may also picture to oneself the matter in this manner, that the ectoderm 

 in its turn becomes invaginated, and that in this way the primitive form of the inhalent 

 canal system originates. From what has been said above, there follows: — (l) that the 

 radial tubes are a kind of flagellated chambers, although not complete homologues of 

 these ; (2) that Sycon being immediately derivable from Ascon, can also change into 

 Leucon ; (3) that the so-called simple Leucon is homologous with a simple Sycon or a 

 simple Ascon, each of the three having the value of an individual (' individuum ' of the 

 third order)." 1 



If I understand Dr. Vosmaer rightly, he considers Leucones and Sycones to be 

 divergent branches of the same bough of the genealogical tree, although not in the same, 

 sense as Prof. Hseckel. It will be proved, by and by, that it is not the case, that Leucones 

 descend from Sycones, still less from a form representing a secondary, not primary, 

 Sycon-type. Independently of this, I completely agree with the first and the third of 

 his conclusions. Like Dr. Vosmaer, I regard the radial tubes as nothing but a kind of 

 flagellated chambers ; I also consider a Sycon as well as a Leucon to be homologous with 

 an Ascon. But though agreeing with these two conclusions, I cannot do so with their 

 postulates. Vosmaer says : " Now it is evidently profitable to the sponge that the surface 

 washed by the water be great, and, considering an eventual extension of the flagellated 

 epithelium-layer to be a favourable factor in the struggle for existence, he tries to explain 

 by it the metamorphosis of an Ascon into a Sycon. This opinion of Dr. Vosmaer has no 

 foundation in fact. He regards a Sycon as better fitted for the struggle for existence than 

 an Ascon. I am forced to remark that had Vosmaer been perfectly logical he would have 

 come to an exactly opposite conclusion. For, compared with an Ascon, a Sycon is more 

 scantily provided with flagellated epithelium : each of its tubes being physiologically 

 equivalent to an Ascon, we have there the whole inner surface deprived of flagellated, and 

 covered with pavement, cells. This is the difference between a Sycon and a colony of 



1 Loc. cit., pp. 159, 160. 



