ILLUSTRATIONS 



PAGE 



55. — Larv^ of Echinoderms (A, B) and of the 



"Acorn Worm" {Balanoglossus) ... 93 



A. Auricularia, larva of a sea-cucumber. 



B. Bipinnaria, larva of a starfish. 



C. Tornaria, larva of Balanoglossus. 

 (A, B, C, after Delage and Herouard.) 



56. — Inner and Outer Mouth Pouches in Embryo 



Vertebrates ...... 94 



A. Larval lamprey, longitudinal section of head. 

 (After Minot.) Showing the nasal pit, hypophysis 

 and mouth cavity arising as infolds from the ectoderm 

 or outer cell-layer. 



B. Embryo rabbit, longitudinal section of head. 



(After Mihalcovics.) 



57. — Attempted Restorations of the Mouth and 

 Gill Region of Two Cephalaspid Ostraco- 

 derms by Stensio ..... 95 



A. Horizontal section through the ventral part of 

 the head of Kioeraspis, showing the assumed position 

 of the gill-sacks. 



The ducts {k. ebr.) leading from the gill-sacks are 

 preserved in the original fossils, also the ridges (z b s) 

 between the ducts, so that by comparison with the 

 anatomy of recent lampreys there is no substantial 

 doubt that the gill-sacks were placed as in the restora- 

 tion. 



B. Underside of the head shield of Cephalaspis, 

 showing the probable position of the gill openings 

 {ebr. c ebr. c) and mouth (m). 



58. — Swift-moving Ostracoderm (Pterolepis nitidus) 



from the Silurian of Norway . . 96 



(After Kiaer.) 



59. — A Modern Descendant of the Ostracoderms 97 



A. Adult lamprey. (After Jordan and Evermann.) 



B. Longitudinal section of larval lamprey, enlarged 

 (After Goodrich.) 



XXV 



