192 



mouth or ingestive aperture marking the line of separation between 

 the outer layer and the inner digestive tract. 



In the Metazoa, we can perceive precisely the same evolution of 

 ingestive organs from the locomotive organs. No known Metazoan is 

 capable of enveloping its prey at any point of its surface, and then 

 conducting digestion there, but the nearest approach to this condition 

 is exemplified by the Coelenterata in many of which the half of the 

 external surface forms an area of polycytic ingestion. 



Thus, in a typical Hydromedusa, (Fig. 11) part of the body-wall 

 is expanded outwards to form an efficient locomotory organ, i. e. the 

 umbrella, the sub-umbrellar cavity thus formed being lined by a layer 



Fig. 11. 



Fig. 12. 



Fig. 11. Diagramm of Hydromedusa, showing locomotive-ingestive area (lo- 

 comotory = black lined) (ingestive = dotted). 



Fig. 12. Diagramm of Scyphomedusa, showing locomotory and ingestive areas 

 indicated as in 11. 



which both structually and ontogenetically belongs to the outer layer. 

 The umbrella serves the double function of locomotion and ingestion 3. 

 Again, in a scyphomedusan such as Aure Ha, (Fig. 12), there is an 

 intermediate condition in which the functions of ingestion and loco- 

 motion are partially separated. Here a portion of the sub-umbrellar 

 cavity is «tucked in«^ and does not assist to any appreciable^ degree 

 in ingestion. 



In a Ctenophore, such as Cijdippe, (Fig. 13) the sub-umbrellar 

 cavity, as is proved by the transitional form Otenaria'^ has been com- 

 pletely invaginated to form what is usually known as the stomach, 

 the true mouth opening from this inwards. The locomotor function 



3 In St. Andrews Museum are Medusae of some 4 inch, diamètre , with no 

 mouth or manubrium. The whole sub. umbr. cavity must here subserve digestion as 

 well as ingestion. 



* Goette, Abhandlungen zur Entwicklung der Thiere, IV. 1886. 



'■> »It seems probable that the stomodaeum in all Anthozoa is simply a food- 

 passage and plays, at most, a very small part in the process of digestion.« S. J. 

 Hickson. Science Progress. 1894. 



^' E. Haeckel, Sitzgsber. Jenaische Gesellseh. 1878. 



