THE GASTKULA 



193 



it necessary that we should most carefully examine the 

 structure of its body. Ordinarily it is invisible to the 



Fig. 24. 



Fig. 25. 



Fig. 26. 



Fig. 27. 



Fig. 23. 



Fig. 28. 



Fig. 23. — (A) 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 (Nauplius). 



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

 flabl.) 



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

 Kowalevsky.) 



In all, d indicates the primitive intestinal cavity; 0, the primitive mouth; 

 s, the cleavage-cavity ; i, the entoderm, or intestinal layer ; e, the exoderm, 

 or skin-layer. 



