498 POLYZOA 



movement takes place in the above cases. The ectocyst of Cris- 

 tatella is confined to the base of the colony, and there forms a 

 thin slimy film, which lubricates the surface over which the 

 animal moves. It has been stated 1 that progression is produced 

 in the following way. The polypides are withdrawn by means 

 of retractor muscles, which originate from the septa and inner 

 surface of the sole. Thus at each retraction of any polypide, the 

 muscle pulls on a portion of the sole. Should the expanded 

 polypides place themselves in a suitable position, the movement 

 will be in the direction of the resultant of the forces due to the 

 separate retractor muscles ; while it is probable that their cilia 

 assist in the onward movement. It should be noted that it is 

 definitely stated that a colony in which all the polypides are 

 retracted can alter its position, 2 although even then the retractor 

 muscles might still contract to some extent. 



The movement probably depends on several causes. It must 

 probably be conceded that the sole itself has some effect on 

 this process. Its outer cells are contractile, and have the power 

 of raising themselves from the underlying ectocyst. They may 

 then again attach themselves, and this new attachment does not 

 always take place in exactly the same place as the former one. 

 Any movement of the muscles of the sole, or of the retractor 

 muscles, will thus shift the skin to a new place. 3 



Protrusion of the Polypide. While it is perfectly clear 

 that retraction is principally performed by the great retractor 

 muscles acting directly on the polypide, it is less easy to explain 

 the converse movement. There can, however, be little doubt 

 that protrusion is effected by the pressure of the fluid of the 

 body-cavity, caused in large part by contractions of the common 

 body-wall. 



Now since, in Cristatella, the body-cavity is a continuous 

 space, any pressure on the fluid must act uniformly on all its 

 contents. The cause which determines the protrusion of a poly- 

 pide is thus to a large extent the relaxation of the sphincter- 

 muscle which surrounds its orifice, aided by special muscles 

 which dilate the orifice. Any polypide which is retracted while 

 the pressure of the fluid in the body-cavity is sufficient to keep 

 other polypides protruded, must therefore keep either its retractor - 



1 Verwom, Zcitsckr. wiss. Zool. xlvi. 1888, p. 119. 2 Dalyell, t. tit. p. 94. 



3 Kraei>elin, Alh. Ver. Hamburg, x. 1887, No. 9, p. 141. 



