DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
801 
evidence on the latter question can be adduced until the development of the axis is 
made known. 
§ 20. Bilateral symmetry of Renilla. 
The very striking bilateral symmetry, both of the individual polyps and of the 
entire community, is constantly brought before our notice in studying the anatomy and 
development of Renilla; and it is impossible to leave the subject without considering 
briefly the significance and origin of this symmetrical arrangement of parts. 
Reviewing the symmetry of the individual, we find that is expressed, firstly, in the 
existence of a dorso-ventral axis, represented by certain median unpaired parts, viz. : 
the elongated oesophagus and mouth, the ventral chamber devoid of a calyx-tooth, 
the dorsal chamber with a well-developed calyx-tooth, and the dorsal and ventral 
median areas of longitudinal muscles. All the remaining parts are bilaterally 
arranged with respect to this axis, viz. : the tentacles, calyx-teeth, septa, mesenterial 
filaments, reproductive organs, and the septal areas of longitudinal muscles. The 
tentacles have a nearly perfect radiate arrangement, but the arrangement of the 
other organs is, to say the least, as much bilateral as radiate. The bilaterality of the 
calyx-teeth is strongly expressed in their mode of development, since, with the exception 
of the dorsal tooth, they appear in successive pairs. The septa are arranged in pairs 
of different lengths, and are joined together at their lower ends in a strictly bilateral 
arrangement. The bilaterality of the mesenterial filaments is nearly as marked as 
that of the septa on account of their arrangement in pairs of different length, their 
structure and rate of development. The reproductive organs have a strictly paired 
arrangement, appearing only on the dorso-lateral and ventro-lateral septa. The 
longitudinal muscles of the septa finally show a marked bilateral symmetry in their 
arrangement, being always placed on the ventral sides of the septa. 
The bilaterality of many of these parts must be of comparatively recent acquisition ; 
for in other and lower polyps it is less evident or entirely wanting. Traces of 
bilateral symmetry are found in nearly all polyps, but in most of the lower forms 
(Zoantharia) radial symmetry, more or less complete, predominates. In the higher 
forms the radiating parts assume a more definitely bilateral arrangement which is 
very marked in the Alcyonaria and reaches its culmination in Renilla* Hence 
there can be no doubt that the bilateral structure is, in part at least, due to 
a rearrangement of parts which were formerly radially symmetrical. The bilateral 
symmetry is, as it were, built upon a basis of radial symmetry ; and traces of the 
latter, more or less pronounced, may accordingly be seen in the' bilateral arrangement 
of most of the parts of Renilla. Thus it exists almost unmodified in the grouping of 
the tentacles, in the septa has partly given place to a bilateral arrangement, and in the 
reproductive organs is scarcely or not at all to be recognised. 
* See especially Haacke, “ Blastologie der Korallen,” Jena. Zeitsclir., Bd. xiii,, 1879, and Haeckel, 
“Generelle Morpliologie” nnd “ Studien z. Gastraea-theorie,” Jena. Zeitsclir., Bd. viii., ix., 1874—5. 
