106 INVERTEBRATE MORPHOLOGY. 



mesenteries, whence they are shed into the intermesenterial 

 chamber and make their exit by the month. The nervous 

 system is well developed especially in the ectoderm of the 

 disk and tentacles, though it also occurs in the endoderm. 

 It possesses the general character of the Coelenterate nervous 

 tissue consisting of sensory cells, nerve-fibres, and ganglion- 

 cells. The muscular system is very well developed both in 

 the ectoderm and endoderm, the muscle-fibres being generally 

 longitudinal in the former layer, and in the latter circular in 

 their direction. At certain regions of the body the muscle- 

 fibres are especially abundantly developed, the mesogloaa 

 being thrown into complicated folds for their support, so that 

 it is possible to distinguish certain definite muscles. One of 

 these is developed upon one face of each mesentery, and, its 

 fibres being directed longitudinally, it forms a strong retractor 

 (Fig. 57, rm) for the disk and tentacles ; a second is developed 

 in the endoderm of the body- wall a short distance below its 

 junction with the disk, and its fibres may, by the growth of 

 the inesogloea around them, become imbedded in that layer ; 

 it forms a more or less powerful sphincter, serving to cover in 

 the disk and tentacles when these have been retracted by the 

 mesenterial retractors. 



The Anthozoa are constructed upon a radial symmetry, 

 as are the other Coslentera, this symmetry appearing in the 

 arrangement of the mesenteries and tentacles and in the cylin- 

 drical form of the body. Nevertheless it is always possible 

 to divide the Anthozoan by a single plane into two similar 

 halves, that is, a bilateral symmetry is also present which is 

 produced by the arrangement of the retractor muscles on only 

 one face of each mesentery and by the flattening of the sto- 

 matodaeum. This latter feature is furthermore usually made 

 more pronounced by the occurrence, at one or both ends of 

 the longer transverse axis of the stornatoda3um, of a distinct 

 groove lined by high columnar cells with long cilia, these 

 grooves forming the siplionogly plies (Fig. 57, si), and by the 

 mesenteries which are attached to the stomatodaeum in the 

 neighborhood of the siphonoglyphes usually having their 

 retractor muscles on different faces from those on which they 

 occur in the other mesenteries. 



