Marvels of Pond- Life. 125 



of some other creatures. The Professsor was never able 

 to discover any mode by which they were permitted to 

 escape from the cells, and in our colonies none were 

 allowed to leave their homes until the death of their 

 parent, and the decomposition of its cell had taken 

 place ; a process which went on contemporaneously with 

 the growth of new cells, until the plant on which the 

 coencecium was situated, rotted away, and then unfortu- 

 nately the whole concern went to pieces. 



The tubes of the Plumatella, and of most other Poly- 

 zoa, are composed of two coats, called respectively 

 endocyst and ectocyst, that is, " inner case " and " outer 

 case." The first is vitally endowed, and exhibits vessels 

 and muscular fibres. The second or outer case is 

 thrown off by the first. It is a parchment-like substance, 

 strengthened by the adhesion of dirt particles, and does 

 not appear to exercise any vital functions, but to be 

 merely a covering for protection. The inner layer 

 terminates in the neck of the bag before described, as 

 exserted when the polypide comes out, and inverted 

 when it goes in. This mode of making a case or sheath 

 by inversion of a bag is technically called invagination, 

 and is readily seen in new and transparent cells. 



The movement of eversion, or coming out, is chiefly 

 produced by the contraction of the endocyst ; while the 

 inversion, or getting in again, is performed by the long 

 muscles, which, when the animal is extended, are seen 

 attached to it like ropes. Upon these muscles Pro- 

 fessor Allman remarks that they are " especially in- 

 teresting in a physiological point of view, as they seem 

 to present us with an example of true muscular tissue, 



