150 rUYLUM ('(!'. I. ENTER A. 



epithelium into the sacs, and thence into the gastric cavity. 

 They find exit by the mouth, but young embryos may be 

 found swimming in the gastro-vascular canals, and also 

 within the shelter of the long lips. 



Variations. --This common animal, hundreds of which 

 may often be found stranded on a sandy beach, is a good 

 illustration of variability. It often exhibits variations, i.e. 

 inborn changes of germinal origin which result in the 

 organism being different from the norm or average of its 

 species. It is normally tetrapartite, but sexpartite, penta- 

 partite, and, more rarely, tripartite forms occur ; and the 

 detailed variations are manifold. 



Life history of Aurelia. The fertilised ovum divides completely, 

 but not quite equally, to form a blastosphere, with a very narrow slit-like 

 cavity. From the larger-celled hemisphere, single cells migrate into 

 the cavity, and fill this up with a solid mass of endoderm. The 

 archenteron arises as a central cleft in this cell mass, and opens 

 to the exterior temporarily by the primitive mouth. During these 

 processes the embryo elongates, the outer cells become ciliated, and 

 the mouth closes. Thus the embryo becomes a free-swimming oval 

 planula. 



After a short period of free life, this planula settles down on a 

 stone or seaweed, attaching itself by the pole where the mouth formerly 

 opened. At a very early stage the mesoglcea appears between the two 

 layers. At the free pole an ectodermic invagination next occurs, an 

 opening breaks through at its lower end, and thus a gullet lined with 

 ectoderm is formed, which hangs freely in the general cavity. During 

 this process there are formed first two and then four diverticula of the 

 general cavity, which are arranged round the gullet above, and open 

 freely into the digestive cavity below. In the gullet region these are 

 separated by broad septa, which are continued into the lower region of 

 the body as four interradial ridges or tasniokv. The tentacles bud out 

 from the region of the mouth, the first four corresponding in position to 

 the four pouches. Interradially above the four septa, four narrow 

 funnel-shaped invaginations arise ; these are produced by the ingrowth 

 of ectoderm, which then forms the muscle fibres which run down the 

 tsenioke (contract the endodennic muscles of Anthozoa). In contrasting 

 this development with that of the hydroid polype, Goette specially 

 emphasises the fact that the radial symmetry is first indicated by the 

 gut pockets, and the tentacles are late in development. Gcette 

 describes a quite similar process of development in certain sea- 

 anemones, and claims to have found there rudiments of septal pockets 

 and ectodermal muscles, thus confirming his view of the intimate 

 relation between the Anthozoa and Scyphomedusre. 



The larva now forms a " Hydra-tuba " or " Scyphistoma " ; it is about 

 an eighth of an inch in height. By lateral budding, or by the forma : 

 tjon of creeping stolons, it may give rise to larva; like itself. The 



