324 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
Although the developmental evidence is not altogether complete, all the facts 
we possess point to the conclusion that the primary six pairs of mesenteries (Protoc- 
nemes) arise practically in the same manner throughout the Ilexactinice and Zoanthm , 
that is, as bilateral pairs (a corresponding mesentery on each side of the principal 
axis), first toward one aspect of the polyp, then toward the other. In the Cerianthece 
only four of the six pairs of protoenemes seem to be developed; the fifth and sixth 
pairs, which often remain incomplete in other Actiniae , perhaps never appear. 
According to van Beneden (1897), however, the development of the primary mes- 
enteries in the Cerianthece does not admit of comparison with that in the two other 
groups. 
In the three divisions the later mesenteries (Metacnemes) are added in a manner 
differing altogether from that followed by the primary mesenteries, the method 
varying in each of the three groups. In the Cerianthece they continue to arise as 
bilateral pairs at what has been regarded as the dorsal aspect of the polyp; in the 
Zoanthece they are added as unilateral pairs (that is, the two mesenteries of a pair 
are adjacent on one side of the axis) at two regions, one on each side of the sulcar 
directives, each pair consisting of a small and a large mesentery; in the Hexactinice 
new mesenteries beyond the protoenemes appear in unilateral pairs, within the 
primary exocoeles of the protoenemes. In the last group they arise either simulta- 
neously or in successive pairs on each side, the succession being either from the 
dorsal to the ventral aspect of the polyp or vice versa. The mesenteries and other 
organs of the Cerianthece and Zoanthece preserve in the adult a strong bilaterality, 
while in most Ilexactinice they usually ultimately attain a biradial symmetry. The 
separation of the three groups is accentuated by other characters given in their 
definitions, but the order of appearance of the metacnemes and their disposition in 
the adult are the characteristics of primary importance. The bilateral mesenterial 
arrangement occurring in the Cerianthece and Zoanthece is representative of a much 
more ancient type of Actinozoan development than is the cyclic plan followed by the 
Ilexactinice. The characteristics of the three types are diagrammatically represented 
in fig. 1, on the text plate opposite. 
The Edwardsice , Ilalcampece , Protactinium, and others have been proposed as 
Actiniarian tribes of equivalent value to the three above mentioned, but as regards 
their mesenteries they merely exhibit one or other of the developmental stages of 
the Ilexactinice , without introducing any new type of mesenterial sequence. Any 
other characters possessed by them are only of subtribal or less importance. 
Formerly regarded as only tribal subdivisions of the Actiniaria, the Cerianthece , 
Zoanthece , and Ilexactinice have now been raised to the rank of Actinozoan groups of 
equivalent value to those of the Alcyonaria and Antipatharia. Carlgren (1898, 1900a) 
has proposed that the terms Ceriantharia , Zoantharia , and Actiniaria , respectively, 
should replace the tribal names above given. Much objection is to be taken against 
using the old familiar and more comprehensive names Zoantharia and Actiniaria in 
such a restricted manner, but rather than introduce any synonymous terms at this 
critical stage of growth of our knowledge of the relationships of the Actinozoa I 
have adopted them in the present paper, though convinced that they should not be 
ultimately accepted. 
