496 EMBKYOLOG-Y OF THE LOWEK VERTEBRATES oh. 



various groups of the animal kingdom. Thus within the limits of 

 the groups Annelida or Mollusca the blastopore in some forms 

 becomes the mouth, in others the anus. No one would doubt for a 

 moment that the mouth opening is homologous throughout these 

 groups yet in one member of the group it can be traced back to the 

 blastopore while in another member it is the anus which can be so 

 traced. In other forms the gastrular mouth simply vanishes away 

 during development and in some of these cases it assumes a 

 curious elongated slit-like form along the mid-neural line before it 

 disappears. 



It is the merit of the Protostoma theory that it — and it alone — 

 affords an explanation of these four very different but equally puzzling 



bodies of facts. It falls there- 

 fore to be accepted by the 

 Vertebrate embryologist as 

 one of his working hypotheses. 

 The Protostoma theory is 

 simply a special development 

 of the theory of the evolution 

 of the coelomate Metazoa 

 which is generally accepted 

 by rnorphologists, namely that 

 the animals in question have 

 passed, during the remoter 

 parts of their evolutionary 

 history, through a Protozoan 

 and later a Coelenterate stage. 

 The peculiarity of the Proto- 

 stoma theoryis that it includes 

 within the coelenterate period 

 a stage corresponding in its 

 main structural features with 

 the Actinians of the present 

 time, characterized by the 

 presence of an elongated slit-like mouth, dilated somewhat at each 

 end and surrounded by a specially concentrated portion of the ecto- 

 dermal nerve plexus. The portion of the surface on which the 

 slit-like mouth was situated was thus the neural surface. 



Sedgwick (1884) was led to the idea by his studies on the 

 development of Peripatus. He found in the species investigated by 

 him a stage (see Fig. 221, A) in which the gastrula-mouth formed a long 

 slit traversing the neural surface and surrounded by the ectodermal 

 neural rudiment. As development went on the gastrular mouth or 

 protostoma became obliterated, except in its dilated terminal portions, 

 by fusion of its lips. The terminal parts remained open as mouth 

 and anus respectively. The portions of nerve rudiment between the 

 two openings became the ventral nerve cords while the portions in 

 front of the mouth and behind the anus gave rise respectively to the 



B 



Fig. 221. — View of neural rudiment in embryo of 

 (A) Peripatus (after Sedgwick, 1884) ; and (B) 

 Lepidosiren. In the case of Lepidosiren the 

 embryo is shown as it appears when straight- 

 ened out. 



