162 LECTURE IX. 



from the ovicapsule of the Campanularia is the equivalent, or homo- 

 logue, of the ciliated planula which, in like manner, escapes from the 

 ovicapsule of the Sertidaria ; and the difficulty or inconsistency 

 cannot be eluded by affirming the bell-shaped medusoid to be a 

 " locomotive ovarium," a mere organ of the compound polype that 

 produced it. 



In the pulmograde Acalephaa the' body is chiefly composed of the 

 "disc," which is circular, almost flat in some, o,.^. JEqtiorea, but 

 rising in others by degrees to a hemispheric form, and becoming 

 nearly cylindrical, as, e.g. in Turris. It is usually smooth, rarely 

 pilose. Around its margin there is, in many species, a projecting 

 ledge of membrane called the " velum," and the margin itself is usually 

 provided with more or less numerous tentacles, and with the " cys- 

 ticles " and pigment specks. From the centre of the under surface 

 of the disc there depends a simple or complex, long or short, process, 

 called the "proboscis," including more or less of the digestive cavity. 



The most characteristic features recognisable by the naked eye, in 

 the organisation of the Acalephte, may be exemplified by the anatomy 

 of the larger Medusae of our own seas. 



The first thing which astonishes us in commencing the dissection 

 of these creatures is the apparent homogeneity of their frail gela- 

 tinous tissue ; secondly, the very large proportion of the body which 

 seems to consist of sea water ; for let this fluid part of a large 

 Medusa, which may weigh two pounds when recently removed from 

 the sea, drain from the solid parts of the body, and these, when 

 dried, will be I'epresented by a thin film of membrane, not ex- 

 ceeding thirty grains in weight. The anatomist is bafiled by the 

 very simplicity, as it seems, of his subject, instead of, as in other 

 cases, by the inability to pursue and unravel all the obvious in- 

 tricate combinations of the created mechanism. Peron and Lesueur, 

 two experienced French naturalists, who, during the circumnavi- 

 gatory voyage to which they were attached, paid great attention 

 to the floating Acalephte, have thus summed up the results of 

 their experience in regard to their organisation. " The substance 

 of a Medusa is wholly resolved, by a kind of instantaneous fusion, 

 into a fluid analogous to sea water ; and yet the most important 

 functions of life are effected in bodies that seem to be nothing more 

 than, as it were, coagulated water. The multiplication of these 

 animals is prodigious ; and we know nothing cei'tain respecting their 

 mode of generation. They may acquire dimensions of many feet 

 diameter, and weigh occasionally from fifty to sixty pounds ; and 

 their system of nutrition escapes us. They execute the most rapid 

 and continued motions ; and the details of their muscular system are 

 unknown. Their secretions seem to be extremely abundant ; but we 



