184 THE EVOLUTION OF THE METAZOA 



shaped strings of muscles that can be observed in the sarco- 

 septa have developed from entoderm, while the muscles 

 of the taeniolae of scyphopolyps have developed from 

 the ectoderm. A similar situation can also be found in connec- 

 tion with gonads (endocarpous-ectocarpous). As a matter of 

 fact, however, this is not a fundamental difference. Neither 

 the muscle fibres nor the sex cells should be considered as 

 entodermal or ectodermal, even if they appear one time as 

 appendages of the skin epithelium, and at another time as 

 those of the intestinal epithelium. It should be added that in 

 the large and certainly primarily solitary Cerianthidae no con- 

 centration of longitudinal muscle fibres into flags can be ob- 

 served (cf. brothers Hertwig, 1879:117). Longitudinal muscles 

 appear under the skin in the body wall; muscle fibres that 

 extend somewhat transversally can be found in the proximal 

 parts of their numerous sarcosepta, and it is only in the thick- 

 ened free ends of sarcosepta where stronger strings of longi- 

 tudinal muscles appear. With regard to the latter it could be 

 stated that they are in all probability "ectodermal" in their 

 origin because it is supposed that the ectodermal tissue sinks 

 downwards from the intestinal tube along the free ends of 

 sarcosepta. We find, conspicuously enough, in Cerianthis and 

 in the polyp of Nausitho'e a tendency of the free ends of 

 their septa to fork when they grow into filaments. 



The strings of longitudinal muscles of antho- and scypho- 

 polyps were in all probability even better developed in the 

 now extinct Tetracorallia (e.g. in Calceola); they helped the 

 animal to retract. 



Here may be the right place to try to get a clear under- 

 standing of the tact that a strong tendency to tetramerous 

 radial symmetry can be observed quite generally in the 

 interior (and partly also in the exterior) of Cnidaria, and 

 particularly in Scyphozoa and in Hydrozoa. First I wish to 

 emphasize that this is not a general characteristic of sessile 

 animals as such. There is, to give an example, a pentamerous 

 radial symmetry in Echinodermata which had certainly evolved 



