DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
769 
development is greatly condensed and abbreviated, and shows not the slightest indi¬ 
cation of any such remarkable and regular sequence as that which Lacaze-Duthiers 
has shown to characterise the development of the septa in various representatives of 
the Zoantharia.* In this respect Renilla agrees with all the Alcyonaria whose 
embryonic development has been investigated, though observations on this matter are 
so scanty as to afford no satisfactory basis for comparison. In the case of Alcyonium , 
Kowalevsky was unable to make out the succession of the septa, but he states that it 
seemed to be analogous to that of the Zoantharia, as described by Lacaze-Duthiers. 
This statement is, however, too vague to be of any value. 
The lateral forward extensions of the peduncular septum (fig. 154) have precisely 
the same structure as the ordinary septa, and they are continuous anteriorly w T ith the 
dorsal pair of septa. Hence there can be little doubt that the peduncular septum is 
to be regarded as formed by the union of the dorsal pair of radial septa, beginning at 
the posterior end and extending thence forwards. It is highly probable that all of 
the septa in Renilla originally extended to the posterior extremity of the body; for 
this is the case in the larval Leptogorgia and in nearly all other polyps ( Cerianthus 
excepted). The six ventral septa have ceased to extend as far as the posterior 
extremity, but the primitive condition has been retained by the dorsal septa and they 
have furthermore united by their inner edges to form a flat plate, the peduncular 
septum. 
The axial cells, according to this view, are to be regarded as having been formed 
along the line of union between the two septa by a peculiar arrangement of the 
entoderm cells in this region (see fig. 152). Before considering the cause of such an 
arrangement, it is necessary to look for the homologue of the peduncular septum in 
other Pennatulids. As has already been stated, no homologous structure is known to 
exist, except in the Pennatulacea ; but its homologies in this group appear tolerably 
clear although they cannot be determined with certainty without further embryological 
investigation. Kolliker has described with great care the structure of the peduncle 
in many species of Pennatulids. The most usual and typical structure is as follows. 
The cavity of the peduncle is divided by four septa into four chambers of which two 
occupy a lateral position, the third is dorsal, and the fourth ventral. The four septa 
meet in the middle of the peduncular cavity, forming a central mass within which 
lies the axis enclosed in an epithelial sheath. Toward the posterior end the two lower 
septa become free from the body walls and run out upon the hinder end of the axis 
which lies free in the peduncular cavity. A part of each upper septum likewise 
extends out upon the free extremity of the axis, but the remaining parts of the upper 
septa fuse together to form a single transverse septum which runs backwards to the 
tip of the peduncle and thus divides the latter at its posterior end into a dorsal and 
a ventral chamber. (For a full description of this very peculiar arrangement, which 
can scarcely be described without figures, see Kolliker’s ‘ Pennatuliden,’ p. 23). 
* Arch. d. Zool. Exp., tome i., ii. 
