114 « COMPARATIVE ANATOMY. 



shows that the organisation of the Spongias is not only very vari- 

 able, but also that to understand it, it is absolutely necessary to 

 distinguish clearly between the physiological and the morphological 

 value of an organ. 



§ 89. 



In its earliest characters the formation of the enteric cavity 

 of the Acalephae agrees with that of the Spongiaa, but in the matured 

 condition there are peculiarities in the Acalepha3, owing to the 

 greater regularity in the arrangement of the system, which is 

 developed out of a simple cavity. The mouth, the extent of which 

 is often increased by the development of accessory parts in its 

 neighbourhood, leads into the digestive cavity, and serves also as an 

 opening for the excretion of undigested matters. The principal 

 cavity seldom remains single, but grows out into secondary cavities, 

 which have the character of pouches, or of canals, and which also, 

 as a rule, correspond to physiological differences, since by them the 

 chyme which is contained in them is distributed through the body 

 of the person, and of the stock. These accessory spaces of the 

 digestive cavity, included with the latter under the designation 

 " gastrovascular system," undertake the function of a circulatory 

 system, without being, morphologically, anything else than the dif- 

 ferentiations of a primitive enteric cavity. The gastric system of 

 the Acalephaa agrees therefore genetically with that of the Spongia3, 

 but is distinguished from it by the exhibition of a higher differentia- 

 tion. This is seen in the difference between the accessory spaces 

 and the central primary one, which forms the stomach, to which its 

 functions are ordinarily limited, and which are not, as in the Spongire, 

 handed over to the secondary spaces. 



§ 90. 



The simplest form of the gastrovascular system is found in the 

 Hydroida. In Hydra it forms a sjDace traversing the long axis of 

 the body, which commences with the mouth, in the middle of the 

 circlet of tentacles, and is continued from the next portion, the 

 stomach, which is capable of great extension, into the thinner 

 portion of the body, where it is narrower. This space is also con- 

 tinued into the tentacles. In the Hydroid-Polyps which form 

 colonies, the canal which arises from the stomach runs through the 

 whole stock, and makes the gastrovascular system common to all the 

 persons. In the stocks of the Siphonophora, some persons only are 

 set apart for the ingestion of nutriment. Each corresponds in 

 structure to the stomachal tube of a Medusa, and forms a tube 

 capable of great extension, which is connected at its base with the 

 general cavitary system of the stock. In this case, then, we must sup- 

 pose that this sort of individuals has lost all the arrangements found 

 in the body of a Medusa, with the exception of the stomach (§ 75). 



The gastric system of the Medusas (Hydromedusas as well as 

 Discophorn) presents numerous variations. It always occupies the 



