Introduction to Animal Morphology. 



exhibit a double contour, due to the formation of an outer, 

 thin, chitinous (?) cuticle, between which and the central 

 mass a fluid layer is interposed. The ovum now is set free, 

 and the central yelk cells fuse into a plasmodium, which 

 divides into a clear ectoderm and a darker endoderm. The 

 horny cuticle splits, and is shed ; hence the adult has no 

 cuticle. The germ becomes ellipsoidal, clear at one pole, 

 where by gradual thinning it develops a mouth, surrounded by 

 4-7 tubercles, which become tentacles, into which the endo- 

 derm is prolonged. The embryo has thus no free ciliated 

 stage. 



Hydrae thus produced never develop sex-organs, but 

 multiply by gemmation or fission for several generations be- 

 fore new sex-products are formed. We have thus a perfect 

 alternation of generations. 



T<> this group belong the voracious freshwater Hydra 

 viridis Fi^. 1 1, 1>), aurantiaca (Fig. 10), grisea, &c., differing 

 in colour, size, and lengths of tentacles. 



Trotohydra Leiickarti (Grcff) a marine, non-tentaculated 

 form, found in the diatom slime of the Ostend oyster park, 

 also belongs here ; it has no distinct cell-membrane, nor 

 pigment in its ectoderm, but large cnidce and a cuticle. 

 Neither sexual nor gemmative reproduction have been found, 

 but it multiplies freely by fission. It may be a larval form. 



CHAPTER XIII. 



HYDROZOA. 



ORDER 2. Gymnoblastea (Allman}. In the last 

 group, continuous aggregation is a temporary con- 

 dition, but in the other orders the colony is the 

 commonest state of existence. Each colony has a 

 root (hydrorhiza) attached to some sub-aquatic body ; 

 from this arises a simple or branched tubular, 

 ccenosarcal stem (hydrocaulus). These two parts 



