IXTKODUCTIOX. XXIX 



Of the tissue described by Fritz Miiller as a common 

 or " colonial " nervous system, an accovmt Avill be given 

 hereafter. 



Muscular System. — This is largely developed amongst 

 the Polyzoa, and exhibits a high degree of complexity. 

 Nor will any one be surprised at this who has observed the 

 rapid and varied movements of the polypide and the curi- 

 ous mechanism of the cell. For the mere act of retraction 

 a whole apparatus of muscles is required ; and, in addition 

 to this simple movement, the remarkable mobility and 

 varied play of the corona as a whole, and of the individual 

 tentacles, involve a special service. The opercular mecha- 

 nism demands a distinct set of muscles; another is needed 

 to assist in the withdrawal and stowage of the alimentary 

 canal, another to steady it and hold it in its place when 

 extended, and yet another to secure the contractility of 

 the inner wall, on which the protrusion of the polypide is 

 largely dependent. In addition to all these claims, the 

 various regions of the digestive system must have their 

 own supplies of contractile tissue to enable them to dis- 

 charge their respective functions efficiently. The power 

 of rapid retreat is essential to the safety of the poly- 

 pide; and hardly less essential is the perfect mobility 

 of the tentacular wreath ; and no one can have watched 

 a living colony without seeing how admirably these are 

 provided for. At the slightest alarm the exquisite bells 

 collapse and vanish on the instant. When the polypide 

 is expanded and in quest of food the corona can be 

 swayed in every direction, and each of the tentacles 

 composing it can be brought into varied and vigorous 

 action. 



