METAZOA COELENTERA. 103 



so close to the surface that one may doubt its existence. 

 Singularly enough Protohydra increases by cross division. 

 PI. 125, fig. i shows the beginning of the process. The 

 constriction goes on until the two are nearly ready to 

 separate" (figs. 2,3) when the two break away and live 

 independent lives. 



This process of reproduction may indicate affinities 

 with the Protozoa, some of which, as we have seen, 

 increase by transverse division, or it may be possible that 

 Protohydra is a reduced form of the Discophora, a group 

 which multiplies by cross division with alternation of gen- 

 erations, and one which will be described farther on. 



We consider Hydra as a reduced form although most 

 of the books place it as a primitive hydroid. Observa- 

 tions 1 have shown that the ectoderm of the embryo of 

 Hydra excretes a chitinous coat. This is probably the 

 vestige of the horny covering or exoskeleton of the Tubu- 

 larian hydroids. Later this sheath is thrown off. 



We have already seen that primitive forms are, as a 

 rule, marine, and the Hydra has been found but once in 

 brackish "water, 2 being preeminently a fresh-water animal. 

 The middle layer which exists under varying forms in the 

 Coelentera consists in the Hydra of many delicate fila- 

 ments extending from the cells of the ectoderm. These 

 may represent either rudiments or vestiges of cells and 

 in the present state of our knowledge it is impossible to 

 say which they are with absolute certainty. It would 

 seem, however, from observations already made, that they 

 are vestiges and that we are not dealing here with a prim- 

 itive adult two layered* animal, whose proper taxonomic 

 position would be before the sponges, but rather with a 

 modified and reduced hydroid form. 



1 See Kleinenberg, The Hydra, 1872. Huxley, Anatomy of 

 Invertebrate Animals, 1878, p. 121. Korotneff, Embryology of the 

 Hydra, Zeitschr. f. wiss. Zool., XXXVIII, 1883, pp. 314-321. 

 Brauer, Zeitschr. f. wiss. Zool., LII, 1891 ; abstract, Journ. Roy. 

 Micr. Soc., 1891, p. 609 ; also Amer. Nat., XXV, 1891, p. 1027. 



2 Stand. Nat. Hist., I, 1885, p. 77. 



