ii CNIDARIA MUSCULATURE 89 



The longitudinal muscles of the septa show a great analogy with 

 the septal or peduncle muscles of the lower Acraspeda. The former 

 appear, however, according to present knowledge, to be endodermal 

 muscles. 



The muscular elements of Corals are either epithelial muscle cells 

 (endodermal musculature), or sub-epithelial muscle cells (ectodermal 

 musculature), or mesodermal muscles (in some species at different parts 

 of the body). 



The following applies to the musculature of the Medusae, Siphonophora, and Corals. 

 If it is much developed in one part, the muscle lamella lays itself, for the purpose of 

 superficial increase, in parallel folds like the leaves of a book (Fig. 71, C]. These folds 

 may again fold themselves in a more or less complicated manner, so as to have a 

 feathered appearance in transverse section. 



A c 



mt, 



FIG. 71. Diagrammatic representation of the various arrangements of the ectodermal 

 Cnidarian musculature in transverse sections through the body wall, e, Ectoderm ; en, endo- 

 derm ; m, muscle lamella ; mz, cell bodies of the muscle fibres ; sm, supporting membrane, jelly. 



The mesodermal supporting substance or supporting membrane takes part in the 

 folding of the contiguous muscle lamellae by itself running within the folds in the 

 form of lamellae. 



When muscle folds completely detach themselves from their matrix, the epi- 

 thelium, and when the free edges of each fold coalesce, there arise out of these sub- 

 epithelial muscle folds mesodermal muscle tubes (D} t which are then surrounded on 

 all sides by the supporting substance. 



In the musculature of the Ctenophora, we in the first place dis- 

 tinguish an ectodermal and a mesodermal portion, between which, 

 however, no very sharp boundary can be made. The ectodermal muscu- 

 lature consists of elongated sub-epithelial fibres on the boundary be- 

 tween the epithelium and the jelly; these may have very various 

 courses. Sometimes we can more or less clearly distinguish a system 

 of longitudinal from a system of circular fibres. The ectodermal 

 musculature is also continued on the oasophagus. The mesodermal 

 musculature, which lies in the jelly, is more strongly developed than 

 the ectodermal musculature ; its fibres, which have an isolated course 

 and are elegantly branched at each end (Fig. 47, g, p. 47), run in 

 various directions, though usually radially, being stretched between 

 the different parts of the gastro-canal system and the outer integument. 

 The contraction of the tentacles and their lateral filaments is brought 



