558 T. A. STEPHENSON 
the history of the Mesomyaria. It provides an idea parallel 
to that of loss of acontia by various forms, advocated in 
Part: 1. 
I do not feel that the loss of ciliated tracts by some forms 
can be very fully accounted for, but it is easier to explain than 
their independent acquisition in three cases would be. ‘The 
suggestion I should like to offer in this connexion was made 
to me by Professor Fleure, and does seem to make it intelligible. 
In certain Gastropods where the gill-lamellae are not much 
strengthened and kept apart skeletally, there is a device for 
keeping open chinks between them, for the passage of water, 
by means of pads of cilia. It is an attractive idea that part 
of the function of the anemone’s ciliated tracts is something 
of the same sort—a preservation of chinks allowing access of 
water between the mesenteries, for respiratory purposes and 
soon. In the light of this several things may be noted. Among 
the forms with no ciliated tracts there is little or no sphincter, 
which means not much tight closing-up of the body. The 
forms with the tracts have above all developed strong retractors 
or sphincter, or both (with fairly numerous exceptions), and 
can often spend a good deal of time tightly shut up—in which 
condition, of course, the pads would function very well. The 
marked development of the tracts in Zoanthids fits im with 
this idea. Among the tractless forms the only really successful 
ones are the skeleton-making corals, and these have got over 
any difficulty by keeping their mesenteries apart with septa ; 
and the other groups are seemingly quaint survivors, and some 
of them are so constituted that there is not much crowding 
in the coelenteron. It is not impossible that certain appear- 
ances in some of the filaments devoid of ciliated tracts represent 
vestiges of them; similar appearances may be present, it is 
true, in forms with the tracts—but even here they might be 
vestiges of the weak tracts of the ancestor which were super- 
seded by much better ones. On the other side of the question 
it must not be forgotten that there are analogues of the ciliated 
tracts in Ceriantharia, but here again the ancestor may not 
have been far from that of the Zoanthactiniaria. 
