PREVIOUS INTERPRETATIONS OF CNIDARIA 119 



longitudinal fibres has been sufficient for the quick and com- 

 plete retraction of the hydranths; and this was the reason why, 

 besides the taeniolae, the inner retractors of Scyphopolyps have 

 also been lost. The same situation has also been preserved in 

 their medusae even in those cases when these have grown 

 larger (Q.g.Aequorea). Contrary to this situation in Scyphome- 

 dusae, we can find that the exumbrella of Hydromedusae does 

 not show any layer of muscular fibres w^hile at the same time 

 these are strongly developed in the subumbrella where they 

 even begin to form folds (e.g. in Gomonemus); in this they are 

 helped by the velum, a speciality of Hydromedusae, as well 

 as by the firm stratum of a jelly-like substance that can be 

 found in the umbrella. 



According to our interpretation, a thin longitudinal stra- 

 tum is the only thing that has been preserved in Cnidaria, 

 that have adopted the sessile way of life, of the cutaneous muscle 

 tube which had consisted of three layers in their turbellarian 

 ancestors. The inner transversal layer of fibres which is attached 

 to the entodermal epithelium obviously goes back to the mus- 

 cularis of the intestinal tract of Turbellaria, regardless whether 

 in Turbellaria this stratum of muscular fibres (Bresslau, 1933, 

 K.C. Schneider, 1902) have a transverse or a longitudinal and 

 transverse position, or whether these be independent muscle 

 cells, or only appendices of the entodermal epitheUum cells. 

 We have here, moreover, a change of the function inasmuch as 

 in Turbellaria the muscularis exclusively serves the intestinal 

 peristalsis, and in Cnidaria where it works as a part of the ge- 

 neral system of the body muscles, in coordination with the 

 external longitudinally placed layer of fibres. In contractions of 

 the animal body— which are very important for animals that 

 have adopted a sessile way of life because they are enabled by 

 such contractions to liberate themselves quickly from con- 

 tact with a foe or from an unfavourable effect of an environ- 

 ment—the main function lies in the longitudinally placed 

 muscle fibres. During the stretching or extension which 

 follows a contraction when the animal searches for food,. 



