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THE POPULAR SCIENCE MONTHLY. 



The most remarkable fact in regard to these medusae is, that the 

 immature form shows a higher type, a greater differentiation of organs, 

 than the parent hydroid. The medusa possesses, in common with the 

 parent, a digestive cavity and cnidse ; and, in addition to these, an 

 organ at the base of each tentacle, which, if it does not unite within 

 itself the senses of sight and hearing, at least is akin to those organs 

 in the lower invertebrates. They certainly possess distinct bundles 

 of muscles and nerve-ganglia, which are not found in the parent form. 

 When the roving medusa has sown its wild-oats, and comes to settle 

 down into a respectable family hydroid, it loses all these advantages 

 belonging to its wandering life, and becomes in its later form identical 

 with the parent ; it returns to the privileges and traditions of its 

 fathers. 



Fig. 10. Campantjlabia dumosa. (Natural Size, and magnified.) 



The huge Mhizostoma, and the beautiful Chrysaora, common to the 

 English coast, Carpenter tells us, ai*e oceanic medusae developed from 

 a small hydroid stem. The embryo emerges in the form of a ciliated 

 ovule, resembling some of the infusoria. One end contracts, forms a 

 foot and attaches itself, the other sends out four tubular offshoots, as 

 tentacles, and " the central cells melt down to form the cavity of the 

 stomach." This hydra-like form multiplies in the ordinary way by 

 budding, for an indefinite length of time. After a while, however, a 

 change takes place, the stem shows constrictions, beginning near the 

 distal end, till the whole stem looks like a rouleau of coins ; the con- 

 strictions deepen, making the stem look like a pile of saucer-shaped 

 bodies; the disks become serrated, and finally the tentacles which 

 belonged to the original medusae disappear, and new tentacles are 

 formed upon the uppermost disk of the pile. Soon this disk begins to 

 show a sort of convulsive strusrfifle which results in its freeing itself, 

 and swimming away as a medusa; each disk develops in the same 

 way, and in turn separates itself from the parent stem. The original 



