NUTRITION 



young embryos or larvae of many of the lower animals, 

 and showed that the manifold and apparently very 

 different embryonic form of all the higher animals 

 may be reduced to the same common type. To this I 

 gave the name of the "cup-embryo" or gastric larvae 

 (gastrula), and concluded, in virtue of the biogenetic 

 law, that it is the palingenetic reproduction of a cor- 

 responding ancestral form (the gastrcca) maintained 

 until the present by heredity. It was not until much 

 later (1895) that Monticelli discovered a modern gas- 

 traead (pcmmatodiscus) which corresponds completely to 

 this hypothetical ancestor (see the last edition of my 

 Anthropogcny, fig. 287). The simplest living forms of 

 the sponges (olynthus) and the cnidaria (hydra) only 

 differ from this hypothetical primitive form of the 

 gastraea by a few secondary and subsequently acquired 

 features. 



The classes of the lower animals which we comprise 

 under the name coelenteria (or coslenterata in the widest 

 sense) generally agree in having all the functions of 

 nutrition accomplished exclusively (or for the most part) 

 by a single system of organs, the gastro-canal or gastro- 

 vascular system. From their common stem-group, the 

 gastra^ads, three different stems have been evolved — the 

 sponges, cnidaria, and platodes. All these coelenteria 

 have three features in common: (i) The gastric canal 

 or tube has only one opening — the primitive mouth, 

 which serves at once for admitting food and ejecting 

 indigestible matter; there is no anus; (2) there is no 

 special body-cavity (cooloma) distinct from the gastric 

 tube; (3) there is also no trace of a vascular system. 

 All cavities that are found in these lower animals be- 

 sides the digestive gut-cavity are direct processes from 

 it (with the exception of the nephridia in the platodes). 



While the simple digestive gut is the sole organ of 

 nutrition in the stem-group of the gastraeads, we find 



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