LINNEAN SOCIETY OF LONDON. XXIX 



had already * directed attention to the fact that all these types pass 

 in their development through one and the same remarkable form, 

 to which he gave the name of Gastrula, and vphich he regards as the 

 most important and significant embryonal form of the whole animal 

 kingdom. This Gastrula consists of a multicellular, usually oviform, 

 uniaxial body enclosing a simple cavity, the primordial stomach or 

 intestine-cavity, which opens outward on one pole of the axis by a 

 simple orifice, the primordial mouth, and whose walls are composed 

 of two layers — the endoderm or inner germ-lamella, and the ectoderm 

 or outer germ-lamella. 



This larval form has now been shown, by the researches of 

 Haeckel, Kowalevsky, Ray Lankester, and others, to occur in members 

 of all the six higher primary groups of the animal kingdom ; and 

 Haeckel, in conformity with what he has called the biogenetic fun- 

 damental lawf (the recapitulation of ancestral forms in the course 

 of the development of the individual), had already in a former 

 work;}: concluded in favour of a common descent of all the six higher 

 types from a single unknown ancestral form, which must have been 

 constructed essentially like the Gastrula, and to which he gave the 

 name of Gastrjea. 



From this common descent the Protozoa alone are excluded, these 

 not having yet attained to the formation of germ-lamellse or of a true 

 intestinal cavity. 



He regards this difference between the development of the Pro- 

 tozoa and that of all the other animal types as so important that he 

 founds thereon a fundamental division of the whole animal kingdom 

 into two great primary sections — the Protozoa and the Metazoa. 

 The former never undergo segmentation, never develop germ-lamellae^ 

 and never possess a true intestinal cavity ; the latter, which includes 

 all the other types of the animal kingdom, present a true segmen- 

 tation of the egg-cell, have all two primary germ-lamellae (endoderm 

 and ectoderm), a true intestine formed from the endoderm and a 

 true epidermis from the ectoderm ; they all pass through the form 

 of the Gastrula or an embryonic form capable of being immediately 

 deduced from it, and (hypothetically) are all descended from a 

 Gastrsea. 



The only Metazoa which in their existing condition have no 

 intestine are the low worm-groups Cestoda and Acanthocephala ; 

 but these form only an apparent exception ; for the loss of their 



* Die Kalkschwamme, 1872. t Qenerelle Morphologie. 



t Die Kalkschwamme. 



