CLASSIFICATION OF ACTINIARIA 505 
pairs. This, taken among other things, seems to mark them 
off pretty well from other primitives, and constitutes a character 
upon which one is inclined to lay more weight than has been 
done hitherto—it is another, though a less important one, 
the value of which has been somewhat overshadowed as in 
the case of the ‘ ciliated tracts’, by the discussion of ecto- 
dermal musculature. A diagram showing this type of mesen- 
terial arrangement for comparison with others may be found 
in Text-fig. 16, B. 
4. Sprcrat Discussions AND OUTLINE oF NEW SCHEME. 
§A. The Gonactiniidae. 
This family has been made to include Protanthea, 
Gonactinia, Oractis, and Boloceroides. For pur- 
poses of this discussion we shall limit it to Gonactinia and 
Protanthea, with Oractis as a probable but insuffi- 
ciently-known member. Boloceroides requires separate 
treatment. The Gonactiniidae, then, have in common a number 
of characters, most of them primitive. The smooth unspecial- 
ized body has a definite attachable basal end, but without any 
basilar muscles. ‘The animal is small and delicate, and has 
both the inner and outer surfaces of the whole of its mesogloea 
covered by a weak generalized muscle-layer, not specially 
concentrated to form definite retractors or sphincters, and 
present in ectoderm of body-wall and actinopharynx as well 
as elsewhere. The body-wall ectoderm also shares the character 
of that of the tentacles in that it possesses spirocysts. The 
mesenterial filaments are without ciliated tracts, and only 
the first eight mesenteries to appear (i.e. the protocnemes, 
which arise as bilateral couples and not as pairs) are perfect 
(see Text-fig. 16,8). These undifferentiated forms seem to 
come nearer than any surviving thing to the probable ancestor 
of the Zoanthactiniaria (Text-fig. 16, 4), which, whatever it 
was, must surely have had in common with them the small 
size and delicacy, the generalized musculature and generalized 
distribution of spirocysts, and the eight perfect mesenteries 
