COELENTERATA. 



105 



In some sea-anemones contractile fibres the acontia arise from 

 the edges of the lower ends of the mesenteries : they are closely 

 set with thread -cells and can be protruded from the lateral pores 

 (cinclides) in the contraction of the polyp and serve as weapons 

 of defence (Fig. 87). 



The muscular system is much more complicated than in Hydroids. 

 The muscles are both ectodermal and endodermal. The ectodermal 

 muscles of the body- wall (longitudinal) are generally feebly developed, 

 while those of the peristomial disc (radial) and of the tentacles 

 (longitudinal) are powerfully developed. The endodermal circular 

 muscles of the pedal disc, the side body-wall, and of the oesophageal 



FIG. 88. Transverse section through 

 an Alcyonarian (after R. Hertwig). 

 11 gonidial groove ; 1, 2, 3, 4 the 

 four pairs of mesenteries with their 

 muscles. 



FIG. 89. Section through an Actinian (Adamsia), 

 after R. Hertwig. Hf the unpaired (dorsal and 

 ventral) chambers ; R, R gonidial grooves. 



tube are well developed. To this system there is added a character- 

 istic and well-developed set of endodermal muscles on the mesenteries. 

 Each septum is provided on one face with transverse (radial) fibres, 

 and on the other with longitudinal (Fig. 90). The lowest section of 

 the transverse muscles are often independent of the rest, and pass 

 from the side body-wall to the pedal disc (m.t). The longitudinal 

 muscles are well developed and cause a projection on the face of the 

 mesentery (Figs. 88, 89). While a nucleus is generally associated 

 with each of the ectodermal muscular fibres, the endodermal muscles 

 are in close connection with the base of a cylindrical epithelial cell. 



In correspondence with the powerful formation of the musculature, 

 the nervous system reaches a considerable development. It has the 



