File:The Journal of microscopy and natural science (1896) (14779043581).jpg

Identifier: journalofmicrosc15post (find matches)
Title: The Journal of microscopy and natural science
Year: 1896 (1890s)
Authors: Postal Microscopical Society Wesley Naturalists' Society Allen, Alfred
Subjects: Microscopy Microscopes
Publisher: (London : W. P. Collins
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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20). The individuals of such stocks are,however, generally not similar, but, as a consequence of more orless division of labour, Dimorphism or Polymorphism takes place.We distinguish (i) sterile ?iutritive persons, which remain on thelevel of the Hydroid, and undertake the feeding of the stock, thegastric cavities of the individuals of the stock being in communi- COMPARATIVE ANATOMY. 279 cation with one another ; (2) Sexual persons, which undertake theduty of ripening the sexual products and also of planting themout and dispersing them, so that the young brood of Hydroidsproceeding from the fertilised egg may attach themselves in newplaces and form new stocks. lhe sexual persons which are des-tined for a free-swimming life, and which are buds of the Hydroidstock, attain a structure corresponding with this manner of life ;they become young Craspedote Mcdusce, which detach themselvesfrom the stock, swim away, and—often after longer or shortermetamorphosis—ripen the sexual products.
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Fii^. 20.—Bougainville ramosa (after Allman), with budding Medusse.h. Nutritive polyps; vi.k.. Medusa buds; w., Detached young Medusa (Margalis ramosa). Turning now towards the end of Vol. I., we come to ClassAfitennata, sub-class Hexapoda or Insecta, which is thus described : The body of the Hexapoda falls typically into three parts, quitedistinct from each other : head, thorax, and hind body (abdomen).The unsegmented head probably originally consisted of four seg- 280 COMPARATIVE ANATOMY. ments. The thorax is composed of three segments : prothorax,mesothorax, and metathorax, answering to the three anterior trunksegments of the Myriapoda. The typical number of segments inthe hind-body is ten or eleven. The thorax and the abdomentogether form the trunk, which may be compared with the trunkof the Symphyla. Among the Apterygota the Thysaiiura possessten abdominal segments, and the CoUembola a varying number, butalways less than ten. In the Pterygota the number of abdominalsegments in

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