544 



DEVELOPMENT OF MEDUSAE FROM POLYPES. 



Fig. 278. 



Thaumantias pilosella, one of the 

 'naked-eyed' Medusae: — a a, Oral Ten- 

 tacles ; b, Stomach ; c, Gastro-vascular 

 Canals, having the Ovaries, d d, on 

 either side, and terminating in the Mar- 

 ginal Canal, e e. 



on which they may not be found, especially on a calm warm day, by 

 skimming the surface of the sea with the Tow-net (§ 177); and they 

 are capable of being well preserved in Goadby's Solution. 



422. When we turn from 

 these small and simple forms 

 to the large and highly- 

 developed Medusce which 

 are commonly known as 

 'Jelly-fish,' we find that 

 their history is essentially 

 similar ; for their progeny 

 have been ascertained to de- 

 velope themselves in the 

 first instance under the 

 Polype-form, and to lead a 

 life which in all essential 

 respects is Zoophytic ; their 

 development into Medusae 

 taking-place only in the 

 closing phase of their exist- 

 ence, and then rather by 

 Gemmation from the original 

 Polype, than by a meta- 

 morphosis of its own fabric. The Embryo emerges from the cavity 

 of its parent, within which the first stages of its development 

 have taken place, in the condition of a ciliated ' Gemmule, ' of 

 rather oblong form, very closely resembling an Infusory Animal- 

 cule, but destitute of a mouth. One end soon contracts aud 

 attaches itself, however, so as to form a foot ; the other enlarges 

 and opens to form a mouth, four tubercles sprouting around it, 

 which grow into tentacles ; whilst the central cells melt-down to 

 form the cavity of the stomach. Thus a Hydra-like Polype is 

 formed, which soon acquires many additional tentacles ; and this, 

 according to the observations of Sir J. G. Dalyell, leads in every 

 important particular the life of a Hydra, propagates, like it, by 

 repeated gemmation, so that whole colonies are formed as offsets 

 from a single stock, and can be multiplied like it by artificial 

 division, each segment developing itself into a perfect Hydra. 

 There seems to be no definite limit to its continuance in this state, 

 or to its power of giving origin to new Polype-buds ; but under 

 conditions not yet ascertained, the Strobila (as it is termed) ceases 

 to propagate by ordinary gemmation, and enters upon an entirely 

 new series of changes. In the first place, the body becomes more 

 cylindrical in form than it previously was ; then a constriction or 

 indentation is seen around it, just below the ring which encircles 

 the mouth and gives origin to the tentacles ; and similar constric- 

 tions are soon repeated round the lower parts of the cylinder, so 

 as to give to the whole body somewhat the appearance of a rouleau 



