ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 355 



Fig. 23. — Pluteus. 18 days. X 55. From the dorsal aspect. Irregular green 



pigment appearing in the anus besides the definite masses in the anterior 



epaulettes. 

 Fig. 24. — Pluteus. 20 days. X 55. From the dorsal aspect. The late plutei of 



E. miliaris have typically short arms, and a broad, domed body. 

 Fig. 25. — Pluteus. 18 days. X 100. From the dorsal aspect. This pluteus has 



developed very rapidly. Note absence of posterior ciliated epaulettes 



and of posterior pedicellaria. 



Plate 20. 



Figs. 2G-35.— ^. escnlentits ? X K acutus $ . 



-Gastrula. 65 hours. X 175. Mesenchyme cells and commencement of 



skeleton are visible. 

 -Prism larva. 4 days. X 1 80. From the ventral aspect. Commencement 



of postoral arms. 

 -Pluteus, 9 days. X 100. From the dorsal aspect. Appearance of the 



postero-dorsal arms and of the Echinus -rudiment. 

 -Pluteus. 7 days. X 55. From the dorsal aspect. Commencement of 



preoral arms. Commencement of anterior ciliated epaulettes. 

 -Pluteus. 15 days. X 55. From the dorsal aspect. Anterior ciliated 



epaulettes advancing. Commencement of thickenings to form the posterior 



ciliated epaulettes. 

 -Pluteus. 18 days. X 55. From the left side. Anterior and posterior 



ciliated epaulettes present, the latter as yet small. 

 -Pluteus. 24 days. X 55. From the ventral aspect. 

 -Pluteus. 30 days. X 55. Anterior and posterior ciliated epaulettes and 



posterior pedicellaria present. 

 -Pluteus. 39 days. X 55. From the postero-dorsal aspect. Tvs^o posterior 



pedicellariae developed. 

 -Pluteus. 39 days. X 55. From the dorsal aspect. Two posterior 



pedicellaria}. Posterior ciliated epaulettes almost completely encirchng 



the posterior end of the body. 



Figs. 36-43. — E. acutus % X E. esculentus $ . 



Figs. 36 and 37.— Gastrula^. 72 hours. X 100. 



Fig. 38.— Four-armed pluteus. 6 days. X 53. From the dorsal aspect. 



Fig. 39.— Pluteus. 9 days. X 100. From the dorsal aspect. The antero-lateral 



arms are of the acutus type, as seen in fig. 13. Commencement of the 



postero-dorsal arms. 



2 z 2 



