io8 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



Medusa (Limnocodium) ; in this remarkable creature 

 the cells which line the mouth of the gastric tube have 

 the function of secreting cells, while it is only in those 

 that lie at the opposite end of the tube that the intra- 

 cellular method has been observed (Lankester). 



This power of intracellular digestion is not con- 

 fined to the cells that line the gastric or endodermal 

 cells ; Metschnikoff has observed that some of the 

 organs (nematocalyces) of the hydroid Plumularia 

 may be fed with powdered carmine, when the dust will 

 enter into the substance of the cells of the ectoderm, 

 which, like the endodermal cells of Hydra, have re- 

 tained the power of protruding pseudopodia. The 

 mesoderm, likewise, in the form of the wandering cells 

 of sponges, and in the larvae of Echinoderms, where 

 some of the organs disappear, or are not continued on 

 into the structure of the adult, exhibits this same 

 property. Even in the higher Metazoa the white 

 blood corpuscles have been observed by Koch to have 

 in their midst bacilli which they have taken into 

 their own substance ; and in inflammatory processes 

 large connective tissue-cells may be observed eating up 

 blood corpuscles, carmine granules, and pigment 

 particles. 



The lowest and simplest condition of the wall of 

 the gastric cavity is to be seen in the lowest Ccelenterata, 

 which present a far more primitive arrangement than 

 do most of the sponges ; there is, indeed, hardly a 

 perceptible advance on what is found in the typical 

 gastrula, and such as there is, is due to the presence of 

 the tentacles around the mouth ; the central, or axial 

 sac, lined with endodermal cells, is continued into the 

 tentacles. 



If we bear this arrangement carefully in mind, we 

 shall be able to refer to it the greater number of 

 arrangements which are to be found in the higher 

 Ccelenterates ; we have, in other words, to look for an 



