136 THE ANATOMY OF LWERTEBRATED ANIMALS. 



body acquires the form of a flask with a wide neck. The belly 

 of the flask is the commencement of the umbrella of the bud- 

 ding medusoid ; the neck is its gastric division. The belly of 

 the flask, in fact, continues to widen out until it has the form 

 of a flat cup, from the centre of which the relatively small 

 gastric neck projects, and the bud is converted into an unmis- 

 takable medusoid, attached to the cone by the centre of the 

 aboral face of its umbrella. In the mean while, the gelatinous 

 transparent mesoderm has appeared, and, in the umbrella, has 

 acquired a great relative thickness. Into this, eight prolonga- 

 tions of the gastric cavity extend, and give rise to the radial 

 canals, which become united into a circular canal at the cir- 

 cumference of the disk. The velum, tentacula, and lithocysts 

 are developed, and the bud becomes detached as a free swim- 

 ming medusoid. But this medusoid is very different from the 

 Carmarina from which it has budded. For example, it has 

 eight radial canals, while the Carmarina has only six ; it has 

 solid tentacles, while the adult Carmarma has tubular tenta- 

 cles ; it has no gastric cone, and has differently disposed lith- 

 ocysts. Haeckel, in fact, identifies it with Cunina rhodo- 

 dactyla, a form which had hitherto been considered to be not 

 only specifically and generically different from Carmarina^ 

 but to be a member of a distinct family — that of the ^ginidm. 



What makes this process of asexual multiplication more 

 remarkable is, that it takes place in Carmarinoe which have 

 already attained sexual maturity, and in males as well as in 

 females. 



There is reason to believe that a similar process of ento- 

 gastric proliferation occurs in several other species of ^gi- 

 nidce — ^glneta prolifera (Gegenbaur), Eury stoma ruhigi- 

 nosiim (KoUiker), and Cunina Koll'ikeri (F. Miiller) ; but, 

 in all these cases, the medusoids which result fr^m the gem- 

 mative process closely resemble the stock from which the}" 

 are produced. 



As might be expected, the Hydrozoa are extremely rare 

 in the fossil state, and probably the last animal the discovery 

 of fossil remains of which could be anticipated is a jelly-fish. 

 Nevertheless, some impressions of Medusae, in the Solenhofen 

 slates, are sufficiently well preserved to allow of their deter- 

 mination as members of the group of Hhizostomidm.^ The 



» Haeclcel, " Ueber zwel neue fossile Medusen aus der Familie der Khi- 

 zostomiden." (" Jahrbuch fiir Mineralogie," 1866.) 



