Coelenferate Nature of the Sponges. 97 



our towers and other fortifications. In these considerations I 

 have left the Echinodermata out of the question. They and 

 the Cnidaria are only distantly related ; the two classes have 

 acquired the radiate structure independently of each other, 

 and it shows essential diiFerences in them. If I divide a 

 typical Echinoderm (with the fundamental number five) by a 

 polar plane into two equal halves, so that the plane on my 

 side, as far as the central point, halves a radius, it will halve 

 an interradius on the other side of the central point ; but in 

 typical Coelenterata (with the fundameatal number four) I 

 shall under similar circumstances always halve similar parts. 



There is no doubt that free, moving Coelenterata are more 

 ancient than attached ones, just as all sessile animals, which 

 are only conceivable in water, are descended from swimming 

 forms. The inducement to adhesion lies in the impulse more 

 or less inherent in all animals to save themselves labour and 

 bodily exertion as much as possible, and it could only be 

 given when such a superfluity of food occurred that the 

 animals needed to take absolutely no trouble in seeking it. 

 When we see that the polyps, even when they are adherent, 

 as so many have been for a long time, have retained the 

 tentacles and the radiate structure, we must assume that 

 these are of preeminent importance ; and this I believe to be 

 the case especially with the tentacles, which, together with the 

 urticating organs, play so important a part in the obtaining 

 of nourishment. The other radiality may rather be second- 

 arily retained, perhaps in correlation with the tentacles (as 

 frequently, e. g. in Hydra, the radiate structure finds expres- 

 sion only in those organs) ; at least it is precisely in adherent 

 forms, ^. e. in those without free locomotion, that it is inter- 

 fered with more frequently than in others in favour of an 

 incipient bilateral symmetry (Fungidai, Flabellum^ develop- 

 mental stages of corals, Hydroida, &c.) . In the origination 

 of an incipient bilateral symmetry another incident probably 

 cooperates, at least in part, namely regular currents of water ; 

 an adherent radiate animal when growing in a constant 

 current of water must naturally develop especially one axis, 

 that which lies in the direction of the constant current, for 

 in this way alone it offers the greatest resistance with the 

 smallest expenditure of force (growth-energy). A further 

 consequence of sessility is the possibility of an increased 

 development of the mesoderm, especially the formation of 

 heavy skeletal masses. 



As soon as a change of function took place with the 

 gastro-vascular apparatus, as in the Sponges, as soon as the 

 nourishment was taken up by it, the tentacles^ if they had 



