Coelenterate Nature of the Sponges. 99 



into the gastral space, and " at the periphery of the obliquely 

 truncated oscular area there appear symmetrically arranged 

 quadriradiates " [loc. cit. p. 288) , and, indeed, six in number ; 

 the three rays which lie in the same plane are so placed 

 that the two continuous ones, which constitute a curved axis, 

 embrace the margin of the disk peripherally, while the un- 

 paired one passes perpendicular to them centripetally and 

 radially into the disk itself, and thus this oscular area is divided 

 up into six regularly radiately arranged parts (antimeres). 

 But if we remember what were the causes of the radiate struc- 

 ture it becomes clear that it is a matter of perfect indifference 

 what parts of the animal-body may first show it ; like bila- 

 teral symmetry it may manifest itself in any parts which are 

 not situated in the central axis, and in this case one is as im- 

 portant as another. If the gastrsea-theory be true, if of all 

 the systems of organs, so soon as a further division of the 

 animal-body occurred, the digestive cavity first developed, it 

 is by no means logical to assume that the radiate structure 

 would also have affected it first of all ; perhaps quite the con- 

 trary may have been the case, for the gastral cavity was, in tlie 

 first place, the central and axially situated organ par excel- 

 lence^ and it is much more probable that the displacements 

 and radiate arrangements of the parts in the interest of the 

 maintenance of the equilibrium of a swimming animal will 

 have first of all made their appearance in the parietal parts 

 situated around the stomachal cavity. And what does the 

 developmental history of the recent Coelenterata teach us? 

 That it is of no consequence at all whether the radial arrange- 

 ment of the coelenteric apparatus is brought about by centri- 

 fugal diverticula from the gastral space, or by partition-like 

 centripetally growing processes from the wall. 



In my opinion the characters of the gastro-vascular appa- 

 ratus furnish a still stronger proof of the Coelenterate nature 

 of the Sponges than the radiate structure, which only occurs 

 occasionally, although certainly more frequently than people 

 seem inclined to believe. The radiate structure may become 

 effaced in consequence of very long-continued sessility, just 

 as well as parasitism can eliminate the bilateral symmetry of 

 animals ; and if sessility is capable of completely suppressing 

 an organ so unmistakably important as a gastral cavity, it is 

 not easy to see why the radiate structure, which is of far less 

 importance to adherent animals, should be preserved under all 

 circumstances, or even with special frequency. But the 

 other Coelenterate character, the ramification of the gastral 

 cavity in the developed animal in the form of centrifugal ly 

 running canals opening freely outwards, is retained in a true 



