. Beginners' Zoology . to a more numerous progeny than were the habitsof a group where members worked more independently ofone another. As the sponge is a simple example of the devotionof special cells to special purposes^ the hydroid is aprimitive and simple example of the occurrence of organs,that is of special parts of the body set aside for a specialwork. Fig. 42.—The formation of many free swimming jelly-fishes from one fixed hydra-like form. The saucer-likeparts (h) turn over after they separate and become likeFig. 43 or 44. Letters show sequence of diagrams. A 30 BEGINNERS ZOOLOGY shown
. Beginners' Zoology . to a more numerous progeny than were the habitsof a group where members worked more independently ofone another. As the sponge is a simple example of the devotionof special cells to special purposes^ the hydroid is aprimitive and simple example of the occurrence of organs,that is of special parts of the body set aside for a specialwork. Fig. 42.—The formation of many free swimming jelly-fishes from one fixed hydra-like form. The saucer-likeparts (h) turn over after they separate and become likeFig. 43 or 44. Letters show sequence of diagrams. A 30 BEGINNERS ZOOLOGY shown in Fig How many mature hydranths are seen in the hydrotd40? Why are the defensive hydranths— on the outside of thecolony ? Which hy-dranths have no tenta-cles ? Why not ? Jellyfish. — Alterna-tion of Generations.—Medusa. — With somespecies of hydroids, avery curious thing hap-pens. — The hydranthtJiat is to produce theeggs falls off and be-comes independent ofthe colony. More sur-prising still, its appear-. FiG. 43. — A Jellyfish. ance changes entirely and instead of being hydra-Hke, itbecomes the large and complex creature called jellyfish(Fig. 43). Butthe egg of theJellyfish pro-duces a smallhydra-like ani-mal which, givesrise by buddingto a hydroidyand the cycle iscomplete. The bud (orreproductivehydranth) ofthe hydroid Fig. 44. —a jellyfish (medusa).
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