88 COMPARATIVE ANATOMY chap. 



than the ectodermal. The musculature is most highly developed in 

 those Actinia which have no skeleton. It shows in its arrangement 

 much similarity with that of the Scyphomedusm, e.g. the iMcernaria. 

 We have an ectodermal longitudinal muscle system and an endoder- 

 mal circular muscle system (leaving out of sight for the present the 

 fact that single portions of the musculature detach themselves from 

 the epithelial matrix and become mesodermal). 



I. The ectodermal longitudinal muscular system forms (a) the 

 longitudinal muscles of the tentacles and (6) the radial muscles of the 

 oral disc. This system is wanting in the oesophagus, in the outer wall 

 of the hody, and in the pedal disc. Only in some forms, which possess 

 no septal longitudinal muscles {Cerianthus), there are in the body wall 

 strong ectodermal longitudinal muscles which serve as retractors of the 

 body. 



II. The endodermal circular muscular system extends all over the 

 surfaces of the body, and forms a layer of circular fibres in the body 

 wall, an inner circular muscular layer in the tentacles, a layer of con- 

 centric fibres on the oral disc, and a circular muscular kyer round the 

 oesophagus. 



The musculature of the septa in Corals deserves special atten- 

 tion; it consists of a system of longitudinal and of a system of 

 transverse muscles. The longitudinal muscles run along the septa from 

 the pedal to the oral disc, and serve as retractors of the body. They 

 are mostly so strongly developed that they form longitudinal thicken- 

 ings on each septum in the space between it and the next septum (Fig. 

 66, p. 76). The transverse musculature is less strongly developed. It 

 is attached on one side to the body wall, on the other to the pedal disc, 

 the oral disc, and the oesophageal tube. The transverse muscles lie on 

 one surface of each septum, the longitudinal on the other side. Their 

 relative positions on the difi'erent septa varies in different divisions, 

 and is always very characteristic. There is generally only one plane, 

 which can be made to divide the body in such a way that the arrange- 

 ment of the muscles on the septa on each side of it is exactly similar. 

 This median plane runs in the principal axis in the direction of the 

 flattened oesophagus or the slit-like oral aperture. 



In most Actinia with numerous septa of various ages and sizes, 

 septa of equal sizes always go in pairs. The longitudinal muscular 

 thickenings of such a pair of septa are truned towards each other. 

 The longitudinal muscles are therefore turned towards the space 

 between the two septa of such a pair — the so-called intraseptal space — 

 and the transverse muscle layers are turned towards the space between 

 this pair and the next on each side — the interseptal spaces. 



In those Alctjonaria which have 8 partition walls, the muscular 

 thickenings of the 4 septa which lie on one side of the median plane 

 are all directed towards one side (Fig. 66). 



There are, besides these, other types of muscle and septal arrange- 

 ment in the Corals. 



