570 T. A. STEPHENSON 
They have usually been classed with the anemones, but it 
appears that they are almost identical in structure with 
coral-polyps, but unlike anemones. Their lack of skeleton 
cannot keep them out of Madreporaria, and the transference 
makes the division between the two groups, as regards soft 
parts, more intelligible. They are the Corallimorphidae and 
Discosomidae. 
Returning to the sea-anemones proper, they seem first to 
have experimented with further curiosities, which perhaps 
diverged from the main stock about the same time as the 
corals, or a little later. These experimental forms fall mto two 
sets, with a good deal that is primitive about them, one of 
them resembling as nearly as any surviving form the supposed 
ancestor of the whole Zoanthactiniaria. They are the Pro- 
tantheae (Gonactiniudae) and Ptychodacteae. After this the 
main line attaimed a definitely muscular Halcampa-lke 
stage with well-marked ciliated tracts on its mesenterial 
filaments, and from this point two main lines of divergence 
may be traced, and two lesser lines. Of the subsidiaries, one 
eroup (Athenaria) took to, or simply remained in, a burrowing 
life, and retained a good deal of simplicity ; the other (Endo- 
coelactaria) went off in a curious direction, the reverse of that 
taken by most forms, as regards some details of its mesenteries, 
and possibly gives a clue to the origin of Tetracorallia. This 
group shows one tendency in common with the two main lines 
to be next dealt with—-a general move towards increase in size 
of the individual, especially in diameter, and increase in the 
number of effective organs; with musculature tending to 
change from a few strong retractors on a few mesenteries to 
a larger number of less specialized ones. 
The two main lines both went in for development of a mar- 
ginal sphincter, but otherwise their differences of tendency are 
marked. The Mesomyaria developed mesogloeal sphincters, 
and these, when they have special stinging organs, have 
acontia, never or hardly ever acrorhagi or frills. And although 
diverging among themselves, many of them tend after a time 
to take to deep-water life. In correlation with this they may 
