APPENDIX. 



EXPLANATION OF THE PLATES. 



Plate pacing Title-page. 



Developmental History of a Calcareous Sponge (Olyntlms). 

 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 (Bxoderm) 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 o) ; 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 (r). 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 ; letters as in Fig. 6). 

 This Ascula is the recapitulative form, according to the biogenetic 



