54 



M. AnnANDALE : The Fauna of Brackish Poiids. [VOL. I, 



collar does not appear in any circumstance, and the margin of the 

 disk is broken up by deep furrows into twelve lobes, each containing 

 seven tentacles and every two corresponding to one of the six 

 primary tentacles. As lobulation of the disk is generally re- 

 garded as a character of generic value in the group to which Me- 

 tridium belongs, this is a matter of some importance. It must be 

 noted, however, that the lobulation is not a permanent feature of 

 the species or even of the new race, but only occurs in specimens 

 of the latter which have attained a large size. Probably it is 

 brought about by the nature of the radial muscles and the thinness 

 of the wall. It is not in any way comparable to the shallower 

 lobulation of the disk which characterizes many Actinians, but may 

 be of interest in considering the question of the manner in which 

 such permanent lobulation has come about. 



p,G 2- — Expanded disk of M. schillerianum var. exul. from above, nat. size. Only ihe 

 outermost cycle of tentacles is represented. 



I have already referred to the fact that no fold makes its appear- 

 ance round the disk of full-grown individuals of the var. exul when 

 they are in the act of contracting, and also that contraction takes a 

 different course in such individuals from that followed in the case of 

 younger examples of the same variety or of either young or old ex- 

 amples of the typical form. When a full-grown typical individual is 

 irritated, the whole disk is drawn downwards by the contraction of 

 the contractor muscles, and at the same time, or a little later, the 

 sphincter, by contracting, draws in the upper part of the column 

 above the disk, while the diameter of the disk and the length of the 

 tentacles are reduced by contraction of the radial muscles, and the 

 mouth is tightly closed. The tips of the tentacles are bent inwards 

 in a broad arc. In young individuals of the variety the process 



