94 INVERTEBRATE MORPHOLOGY. 



In some forms, such as Hydractinia, the free-swimming embryo when 

 it settles down becomes converted into a flat plate-like expansion without 

 mouth or tentacles, from which, as a bud, the first hydranth arises. 



If, however, tlie ovum develops directly into a medusa, as 

 in the Trachymedusse and Narcomedusse, the breaking through 

 of the mouth and the formation of tentacles takes place 

 while the embryo is still free-sv^imming, and the stage so 

 produced may resemble closely a free-swimming hydranth, as 

 in Cunoctantha, or may, by the great development of mesoglcea 

 at the extremity opposite the mouth, assume a rather globular 

 form, as in Liriope. As the tentacles develop and the bell 

 becomes differentiated by the extension laterally, as it were, 

 of the embryo, the velum arises at the margin of the bell. 

 At this time the coelenteron is a flattened cavity extending 

 to the margins of the bell, but later it becomes obliterated 

 along four lines, and the obliteration of the cavity extending, 

 the radiating and circular canals and the gastric cavity alone 

 persist, a layer of endoderm-cells sometimes joining them 

 and representing the obliterated portion of the coelenteron, 

 though often this also disappears. 



In the Anthomedusse and Leptomedusse, in which the medu- 

 sae arise by budding from the polyps, the buds are at first 

 tubular outgrowths of the body-wall (Fig. 48, A). The ecto- 

 derm at the tip of the bud thickens, depressing the central 

 portion of the endoderm (Fig. 48, B), and on the appearance 

 of a cavity in the thickened ectoderm, the subumbrellar 

 cavity, the central endoderm pushes out into the cavity, carry- 

 ing with it the ectoderm covering it and forming the manu- 

 brium (Fig. 48, C). In this stage the bud, though stiU lacking 

 mouth and tentacles, is comparable to the polyp stage of the 

 medusa of direct development at least so far as the coelen- 

 teron is concerned, and by processes identical with those 

 occurring in the directly-developing embryo the radiating and 

 circular canals are formed (Fig. 48, E), and on the formation 

 of a mouth at the extremity of the manubrium and the de- 

 velopment of tentacles the medusa is perfectly formed. 



In many cases, however, as already stated, the medusa- 

 buds never reach their complete development, but become 

 sexually mature while still imperfect in form. The stage at 



