278 COMPARATIVE ANATOMY. 



among the persons, we have most Corals; and of stocks with far- 

 reaching division of labour and polymorphism, the Siphonospora. 



" Even in the lowest Metazoa the cell elements are not found 

 scattered in the body without any special arrangement. On the 

 contrary, we find even among the simplest Ccelenterata that they 

 are arranged in two epithelium-like layers, which are closely con- 

 tiguous and form the wall of the body, which is pouch-shaped and 

 provided with an opening. In keeping with the physiological 

 activities of the various cells, the stinging cells and the neuro- 

 muscular cells form the outer layer, while the digesting cells form 

 the inner layer, which is turned towards the pouch cavity — i.e., the 

 gastric cavity. The reproductive cells lie protected in the deeper 

 portions of the outer layer. These two layers, which occur in the 

 development of all Metazoa, are called the Ectoderm and the 

 Endoderm." 



Turning to Class III. — Cnidaria, sub-class Hydrozoa — we 

 read: — " In all Hydrozoa an ectodermal cesophagus is wanting. 

 The mouth leads direct into the endodermal gastric cavity. 

 Gastral filaments are wanting. The sexual products mostly arise 

 from the ectoderm. The sexes are generally separate." 



Asexual reproduction is very common among the Cnidaria, 

 side by side with sexual reproduction. Among the Cteitophora 

 alone it has not been observed. In Hydra we find asexual repro- 

 duction by gemmation side by side with sexual reproduction in 

 adult animals. Buds are formed by hollow outgrowths of the 

 body-wall. These buds grow, and at the distal end a breach is 

 formed — an oral aperture, round which the tentacles arise by 

 means of the new outgrowths. Such buds can detach themselves 

 from the mother-body, or they may in small numbers remain 

 united with it for some time. In the last case small Hydra colo- 

 nies, composed of similar individuals, arise. 



In the same way, elegant and richly-branched colonies arise in 

 most Hydroids (Fig. 20). The individuals of such stocks are, 

 however, generally not similar, but, as a consequence of more or 

 less division of labour, Dimorphism or Polymorphism takes place. 

 We distinguish (i) sterile ?iutritive persons, which remain on the 

 level of the Hydroid, and undertake the feeding of the stock, the 

 gastric cavities of the individuals of the stock being in communi- 



