CNIDARIA 



81 



I To explain the phylogenetic development of this axial skeleton of the 

 )rgonidae, v. Koch (No. 85) has for comparison made use of the 

 iteresting discoveries on Gerardia (Antipatharia, Hexacorallia). These 

 colonies of Gerardia form flat membranoid coverings over foreign 

 bodies, and for this purpose commonly select the axial skeleton of dead 

 Gorgonidse as a support. A lamella of horn, which coats the support, 

 is now secreted by the ectoderm of the lower surface of these colonies. 

 The lamella surrounds the axis of the Gorgonia within it like a sheath. 

 When at length the colony of the Gerardia by growth acquires an extent 

 which stretches beyond the limits of the original support, then out- 

 growths covered with young polyps are produced, into which extend 

 horny skeletal processes, produced by the common basal lamella, but no 

 longer enclosing within them any foreign body. It is seen that here is 

 produced the first trace of an independent free axial skeleton, while 

 the basal plate of the skeleton, which in the higher forms is much 

 reduced and attached to a foreign support, arises from the basal lamella. 



Fig. 35,— Two transverse sections through a polyp of the Alcyonarian type 

 (diagram after v. Koch, from Lang's Lehrb\xc\i); that at the left is at the level 

 of the oesophagus, that at the right at the level of the gastral cavity, ab, plane 

 of symmetry. The ventral side is directed upwards. 



The polyps of the Alcjonaria present a typical bilaterally 

 symmetrical structure, which is evident in the first place 

 from the position of the longitudinal muscles in the mesen- 

 terial septa. Here the plane of symmetry (Fig. 35 ah) 

 passes through two unpaired chambers (gastral pouches), 

 which are distinguished from each other by the fact that 

 the two septa which bound the ventral chamber exhibit the 

 muscle ridges on the sides which are turned toward each 

 other, whereas this condition is reversed in the dorsal cham- 

 ber. On the remaining septa, in fact on all the septa, the 

 longitudinal muscle ridges are so arranged that they face 



recently defended the interpretation of the axis of the Gorgonidse as a 

 mesodermal growth. 



K. H. E. G 



