372 SYDNEY J. HICKSON. 



fibrils remain. If the part marked with an asterisk be 

 examined with a high power, the plexus represented in fig. 35 

 will be visible.^ 



We have evidence, then, of the existence of a nervous plexus 

 in both the ectoderm and endoderm of Alcyonium. It has not 

 been found possible at present to trace it in the ectoderm and 

 endoderm of all parts of the body. 



The occurrence of a fine plexus of cells and fibrils in the 

 mesoglcea was noted in a previous publication (15); and 

 although, owing to technical difficulties, this mesoglceal nervous 

 plexus has not been proved to be connected with the nervous 

 plexus of the ectoderm, it is extremely probable that such a 

 connection does exist. There is no difficulty in tracing its 

 connection with the endodermic nerve plexus. 



The nerve plexus of the mesogloea is represented in fig. 26. 

 The figure was drawn from a section through a part of the 

 colony about one inch from the surface. The difficulty of 

 tracing it nearer to the ectoderm is due to the fact that the 

 only satisfactory preparations showing the plexus are made by 

 cutting sections of the living colony as thin as possible with 

 the free hand, and treating them immediately with osmic or 

 pyrolignic acid. The superficial regions of the colony are, 

 however, so densely crowded with spicules that it is impossible 

 to get them in very thin sections. Sections of decalcified 

 specimens which have been made with a microtome do not 

 show the nerve plexus very satisfactorily. 



Muscular System. — The walls of the coelenteron of 

 Alcyonium are provided with a complete sheath of endodermic 

 circular muscular fibres (fig. 33). This sheath is probably 

 used during life for causing the expansion of the polyps. Any 

 contraction of the circular fibres would cause an increased 

 water-pressure in the coblentera and force the polyps out ; but 

 when the colony is disturbed or artificially wounded, and the 

 great mesenteric retractor muscles are contracted, it forces 



' Herdman (17) has described a number of fusiform, polygonal, and trian- 

 gular cells in the endoderm of the mesenteries of Sarcodictyon. 



