TUBES OF PLUMATELLA. 165 



unfortunately the whole concern went to pieces. 



The tubes of the PlumateUa^ and of most other 

 Polyzoa, are composed of two coats, called respec- 

 tively 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 parchmentdike 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 wlien 

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

 by inversion of a bag, is technically called inTa- 

 glnation^ and is readily seen in new and transparent 

 cells. 



The movement of ever si on ^ or coming out, is 

 chiefly produced by the contraction of the endocyst; 

 while the inversion^ or getting in again, is per- 

 formed by the long muscles, which, when the animal 

 is extended, are seen attached to it like ropes. 

 Upon these muscles Professor Allman remarks that 

 they are "especially interesting in a physiological 

 point of view, as they seem to present us with an 

 example of true muscular tissue, reduced to its 

 simplest and essential form. A muscle may here 

 be viewed as a beautiful dissection far surpassing 



