. Beginners' Zoology . nd give off carbondioxide. Blood vessels areunnecessary for transfer-ring oxygen and food fromcell to cell. Reproduction. - Do you p^^ ^g _ j,^.^^^^ ^^ ^,^^ ^„^,, 3^,.see any swellings upon the face of pondweed. side of the hydra i (Fig. 34, A.) If the swelling is nearthe tentacles, it is a spcrmary; if near the base, it is anovary. A sperm coalesces with or fertilizes the ovum afterthe ovum is exposed by the breaking of the ovary the sperm from one hydra unites with the ovumof another hydra. This is called cross-fertilization. Thesame term is applied to t
. Beginners' Zoology . nd give off carbondioxide. Blood vessels areunnecessary for transfer-ring oxygen and food fromcell to cell. Reproduction. - Do you p^^ ^g _ j,^.^^^^ ^^ ^,^^ ^„^,, 3^,.see any swellings upon the face of pondweed. side of the hydra i (Fig. 34, A.) If the swelling is nearthe tentacles, it is a spcrmary; if near the base, it is anovary. A sperm coalesces with or fertilizes the ovum afterthe ovum is exposed by the breaking of the ovary the sperm from one hydra unites with the ovumof another hydra. This is called cross-fertilization. Thesame term is applied to the process in plants when themale element, developed in the pollen of the flower, uniteswith the female element of the ovule of the flower onanother plant. The hydra, like most plants and some otheranimals, is hermaphrodite, that is to say, both sperms andova are produced by one individual. In the autumn, eggsare produced with hard shells to withstand the cold untilspring. Sexual reproduction takes place when food is. 26 BEGINNERS ZOOLOGY scarce. Asexual generation (by budding) is common witfithe hydra when food supply is abundant. After the bud grows to a cer-tain size, theouter layer ofcells at the baseof the bud con-stricts and theyoung hydra isdetached. Compare thesponge and thehydra in the fol-lowing respects:— many celled,or one celled;obtaining food ;breathing; tubesand cavities;openings; re-production ; loco-motion. Whichranks higheramong the metazoa . The metazoa, or many-celled ani-mals, include all animals except which branch ? Figure 39 is a Diicroscopic view of a vertical section of a hydra toshow the structure of the body wall. There is an outer layer called theectoderm, and an inner layer called the endoderm. There is also a thinsupporting layer (black in the figure) called the inesoglea. The mesogleais the thinnest layer. Are the cells larger in the endoderm or the ectoderm ?Do both layers of cells assist in forming the reproductive bud ? The ecto-derm cells
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