THE GASTRULA 



x 93 



it necessary that we- should most carefully examine the 

 structure of its body. Ordinarily it is invisible to the 



Fig. 24 



Fin. 25. 



Fig. 26. 



Fig. 27. 



Fig. 23. 



Fig. 28. 



Fig. 23. — (.4) Gastrula of a Zoophyte (Gastropliysema). (Haeckel.) 

 Fig. 24.— (B) Gastrula of a Worm (Sagitta). (After Kowalevsky.) 

 Fig. 25.— (C) Gastrula of au Echinoderm (Starfish, Uraster). (After 



Alexander Agassiz.) 



Fig. 26. — (D) Gastrula of an Arthropod (NavpUus). 



Fig. 27. — (E) Gastrula of a Mollusc (Pond-snail, Limnceus). (After Karl 



Babl.) 



Fig. 28.— (F) Gastrula of a Vertebrate (Lancelet, Awphioxus). (After 

 Kowalevsky.) 



In all, d indicates the primitive intestinal cavity; o, the primitive mouth; 

 8, the cleavage-cavity; t, the entoderm, or intestinal layer; e, the exoderm, 

 or skin-layer. 



