726 DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
axial polyp, since the ventral chamber, which bears no calyx-teeth, is always directed 
outwards towards the margin of the disc. Thus the secondary polyps stand at all 
angles from 0° to 90° with the axial polyp ; those at the sides are placed so that their 
dorso-ventral axes form right angles with that of the axial polyp, while those directly 
in front of the axial polyp coincide with it in direction. 
Lastly we may note the structure of the peduncle. Its cavity is divided into a 
dorsal and a ventral chamber by a horizontal partition which is pierced along its sides 
and at its lower extremity with openings by which the chambers are put in communi- 
cation. Both chambers end blindly in front, but communicate by small openings with 
the adjacent polyp-cells. The upper canal communicates with the exterior through the 
exhalent zooid already described. The horizontal partition appears to split anteriorly 
into a dorsal and a ventral plate, between which lies the posterior part of the body— 
z.e., part of the cell—of the axial polyp. 
The structure of the body-wall in the peduncle, where it is most fully developed, 
is as follows (after E1sen). Beginning with the exterior there are: (1) external 
epithelium ; (2) a thick layer of connective tissue containing the spicules ; (3) a layer 
of fibrous connective tissue free from spicules ; (4) longitudinal muscles ; (5) circular 
muscles ; (6) internal epithelium. In other parts of the body the arrangement is some- 
what different since the amount and structure of the connective tissue varies in 
different parts of the body and the spicules are absent from the walls of the free 
portions of the polyps. 
Many of the structural features of Renilla are common to other Alcyonaria. The 
polyps always exhibit more or less of bilateral symmetry in the elongation of the 
mouth, disposition of the septa and septal muscles, grouping of the mesenterial 
filaments and arrangement of the reproductive organs. In all but one or two cases 
colonies are formed by processes of asexual multiplication, and these not uncommonly 
show traces of bilateral symmetry. Among the Pennatulacea bilaterality is always 
more or less marked, culminating in the Rendlaceew, where the symmetry is nearly 
complete. 
The definite relation between the dorso-ventral axes of the secondary and primary 
polyps has been observed in a few other Alcyonaria, but observations on this point are 
very scanty. KO6OLLIKER, in his great work on the Pennatulids, has described some- 
thing similar among the more typical forms, and it is highly probable that further 
investigation would show that in all Pennatulacea definite relations of this sort exist. 
Mosgtry observed in Heliopora and Sarcophyton* that the dorsal sides of the polyps 
face in a definite direction ; and according to Haacxet the polyps of Madrepora have 
a like disposition. As pointed out further on, this matter is one of much theoretical 
interest in connexion with the law of budding in Renilla. 
* Phil. Trans., Vol. 166, 1876. 
+ “ Zur Blastologie der Korallen,” Jena. Zeitschrift, Bd. XIII 
