CNIDARIA 79 



In Eenilla the oesophagus is developed in the form of a solid ectodermal 

 ingrowth, in which a fissure makes its api^earance and opens to the 

 exterior, whereas the development of the inner opening of the oesophagus 

 does not take place until later. 



Fig. 33. — Attached stage of Sympodium covalloiUs (after Kowalevsky kt 

 Mabion). iSc, ectoderm ; en, entoderm; si, Oesophagus. 



By the formation of the mesenterial septa the gastral 

 apace is separated into a central stomach cavity and eight 

 peripheral gastral pouches. At the upper ends of the latter 

 hollow, bud-like elevations now arise, in which we recognize 

 the earliest fundaments of the eight (subsequently pinnate) 

 tentacles, which accordingly owe their origin to simple 

 evaginations of the body-wall. 



The development of the septa, the formation of the oeso- 

 phagus, and even the establishment of the tentacles may 

 take place before attachment. In genei'al, however, the 

 attachment of the swarming larva precedes the formation 

 of these organs. By means of the developmental pi'ocesses 

 mentioned the typical structure of the polyp is established. 

 During these metamorphoses important changes in the struc- 

 ture of the ectoderm take place. By multiplication of the cells 

 this layer becomes changed into a multi-layered epithelium. 

 The secretion of a hyaline gelatinous substance [mesoglcea] 

 now takes place between the cells of the deeper layers, 

 which thus lose their connection with one another and 

 assume more and more spindle or stellate shapes (Fig. 34). 

 By these processes two different layers arise from the 

 primary ectoderm : a superficial one, which from now on 

 preserves the name of ectoderm^ and the cells of which have 

 retained the enithelial continuity, and a lower layer, which 

 assumes more and more the character of a gelatinous 

 connective tissue, and which will be called henceforth 

 mesoderm. This layer accordingly is a product of the 



