THE WONDERS OF LIFE 



forms. The class of crested medusae, or ribbed medusae 

 (ctenophorae), contains glasslike, sea-dwelling cnidaria 

 of a peculiar and complicated build, which probably 

 descend from hydromedusae (or craspedota) . But where- 

 as the latter have very simple gonochoristic structures 

 (four or eight monosexual glands in the course of the 

 radial canals or in the gastric wall), in the ctenophorae 

 the eight hermaphroditic canals run in a meridian arch 

 from one pole of the cucumber-shaped body to the other. 

 Each canal corresponds to a ciliary streamer, and forms 

 ovaries at one border and testicles at the other; and 

 these are so arranged that the eight intercostal fields 

 (the spaces between the eight streamers) are alternately 

 male and female. Still more curious are the hermaph- 

 roditic glands of the highly organized, land - dwelling, 

 and air-breathing lung-snails (pulmonata), to which our 

 common garden snail (arion) and vineyard snail {helix) 

 belong. Here we have a hermaphroditic gland with a 

 number of tubes, each of which forms ovaries in its 

 outer part and sperma in the inner. Still the two kinds 

 of sex-cells lead separately outward. 



In most of the lower and aquatic histona both kinds 

 of sex-cells, when they are ripe, fall directly into the 

 water, and come together there. But in most of the 

 higher, and especially the terrestrial, organisms special 

 exits or conducting canals have been formed for the sex- 

 products, the sexual ducts (gofioductus) ; in the metazoa 

 the female have the general name of oviducts and the 

 male spermaducts (or vasa deferentia). In the vivipa- 

 rous histona special canals serve for the conveyance of 

 the sperm to the ovum, which remains inside the moth- 

 er's body; such are the neck of the archegonium in the 

 cryptogams, the pistil in the phanerogams, and the vagina 

 in the metazoa. At the outer opening of these conduct- 

 ing canals special copulative organs are developed, as a 

 rule. 



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