EXPLANATIOK OF THE FIGURES. 



All the drawings have been made with a camera luci' 

 da. The lenses used viere those of Zeiss, and are referred 

 to in the description of each figure. 



Refere nce Letters . 



a_. Anus. 



amp . Ampulla of the "organe refringent." 



bl . s. Blood-sinus of stolon. 



br . Branchial sac. 



br . 0^. Branchial oi'ifice. 



_c. g. Cavity of rudiment of sexual organs. 



c 1 . Cloaca. 



c3^._^. Cloacal orifice. 



£. p_c. Cavity of pericardial sac. 



jc. t. Cells of test. 



d . t . Dorsal tube. 



a_. jtr. Digestive tract. 



d.w.p£. Thickened dorsal wall of pericardiT-ra, which becomes 



invaginated to form the heart. 



£.br._o. Ectodermal invagination to form the brancnial orific( 



— ■Sl'S-' Ectodermal invagination to form the cloacal orifice. 



ect . Ectoderm, 



en. Endoderm. 



end. Endostyle. 



-70 



