ANIMAL AND VEGETABLE TISSUES. 



17 



and sense organs are totally absent in many organisms, which are 

 looked upon as animals, e.g., in the whole of the Protozoa. 



With such reduction of the internal organs it is easy to understand 

 that the simpler lower 

 animals, such as colonies 

 of polyps and the Sipho- 

 nophora, should often in 

 their outer appearance and 

 the manner of their growth 

 resemble plants, with which 

 they were formerly con- 

 founded, especially when 

 they at the same time 

 lacked the power of free 

 locomotion (Polyps, Hy- 

 droids, figs. 4, 5). In these 

 cases it is as difficult to 

 limit the idea of " indi- 

 viduality" as it is in the 

 vegetable kingdom. 



2. Between animal and 

 vegetable tissues there exists 

 also generally an important 

 difference. While in the 

 vegetable tissues the cells 

 preserve their original form 

 and independence, in the 

 animal tissues they undergo 

 very various modifications 

 at the expense of their 

 independence. Accordingly 

 vegetable tissues consist of 

 uniform cell - aggregates, 

 the individual cells of 

 which have 

 sharply - marked bounda- 

 ries; while in animal tis- 

 sues the cells give rise to 

 extremely different structures, in which the cells as such do not 

 always remain recognisable. The reason for this unlike condition of 

 the tissues must apparently be sought in the different structure of 



retained FlG- 5 -~ Pli y s P nora hydrostatica. Pn, Pneuma- 

 tophore ; S, Swimming-bells ; T, Dactylozooid ? 

 P, polypite or stomach with the tentacles, Sf. ; 

 Nk, terminal swellings on the latter provided 

 with thread-cells ; G, Clusters of gonophores 



