APPENDIX. 
EXPLANATION OF THE PLATES. 
PLATE FACING TITLE-PAGE. 
Developmental History of a Calcareous Sponge (Olynthus). 
Compare vol. ii. p. 140. The egg of the Olynthus (Fig. 9), 
which represents the common ancestral form of all Calcareous 
Sponges, is a simple cell (Fig. 1). From this there arises, by 
repeated division (Fig. 2), a globular, mulberry-like heap of 
numerous equi-formal cells (Morula, Fig. 3; vol. ii. p. 125. 
As the result of the change of these cells into an outer series of 
clear ciliated cells (Exoderm) and an inner series of dark, non- 
ciliated cells (Entoderm), the ciliated larva, or Planula, makes 
its appearance. This is oval in shape, and forms a cavity in 
its centre (gastric cavity, or primitive stomach, Fig. 6 g.), with 
an opening (mouth-opening, or primitive mouth, Fig. 6 0); the 
wall of the gastric cavity consists of two layers of cells, or 
germ-layers, the outer ciliated Exoderm (e) and the inner non- 
ciliated Entoderm (7). Thus arises the exceedingly important 
stomach-larva, or Gastrula, which reappears in the most different 
tribes of animals as a common larval form (Fig. 5, seen from the 
surface; Fig. 6, in long section. Compare, vol. ii. pp. 126 and 
281). After the Gastrula has swum about for some time in the 
sea, it fastens itself securely to the sea-bottom, loses its outer 
vibratile processes, or cilia, and changes into the Ascula (Fig. 7, 
seen from the surface; Fig. 8,in long section; lettersasin Fig. 6). 
This Ascula is the recapitulative form, according to the biogenetic 
