THE GASTEULA 



193 



it necessary that we should most carefully examine the 

 structure (jf 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 Zooijhyte {Gastroi-ihyscma). (Haeckel.) 



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



Fig. 25. — {€) Gastrula of aii Echinoderm (Starfish, Vraster'). (After 

 Alexander Agassiz.) 



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



Fig. 27. — (£') Gastrula of a Mollusc (Poud-snail, Limnceus). (After Karl 

 fiabl.) 



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

 Kowalevsky.) 



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

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

 or skin-layer. 



