THE GASTRULA 



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. 



Flu. 23. 



FIG. 28. 



FIG. 23. (A) Gastrnla of a Zoophyte (Gastrophysema). (Haeckel.) 



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



FIG. 25. (C) Gastrula of an 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 

 fiabl.) 



FIG. 28.- (F) Gastrula of a Vertebrate (Lancelot, Amphioxus). (After 

 Kowalevsky.) 



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

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

 or skin-layer. 



