206 NATURAL HISTORY OF 



is found is called the neural side, tliat opposite to the 

 gangUon is called the hcemal side. 



The muscular system of the Polyzoa is complex and 

 extensive. The principal sets of muscles are the 

 retractor muscles and the parietal muscles. 



The retractor muscles consist of two fasciculi or 

 bundles of fibres which are inserted at the base of the 

 coll, and spinng from the endocyst. These fasciculi 

 pass one on each side of the stomach to the upper part 

 of the oesophagus. On retraction the fibres of the 

 fasciculi separate and lie loosely in the cavity of the 

 zooecium, and by their retraction they pull down the 

 alimentary canal, and consequently the tentacles, and 

 cause the latter to be invaginated and withdrawn below 

 the orifice of the cell. Around the invaginated sheath 

 is a set of fibres called the vaginal sphincter, which 

 closes the endocyst after the retraction of the poly- 

 pide. 



The parietal muscles consist of short fibres running 

 transversely in the endocyst in small groups of two or 

 three fibres each, occupying a veiy large portion of 

 the circumference of the cell. These muscles contract 

 the endocyst, which presses upon the fluid contained in 

 the cavity of the zooecium. The result of this pressure 

 and the accompanying relaxation of the vaginal 

 sphincter is to force out the tentacular corona and 

 sheath. The action of the muscles before described is 

 further assisted by two sets of muscles, one behind the 

 other, placed on the inner surface of the endocyst in 

 the neighbourhood of the sheath. These are called 

 the anterior and posterior parieto-vagiual muscles. In 

 addition to the muscles named are those which serve 

 to expand, rotate, and move the tentacles, and to raise 



