178 LECTURE IX. 



plate. In the circular form called Porpita the plate consists of two 

 lamellae, including a great number of air-canals, the parietes of which 

 are slightly calcified: the prehensile organs are chiefly developed 

 from the margin of the float. In the elliptical Velella the horizontal 

 disc consists of four pieces united together by oblique sutures : from 

 the upper surface of the body-disc there rises a second thin semi- 

 elliptic plate, which is set at the same angle to the disc or deck as 

 the lateen sail of the Malay boat. By means of this plate the little 

 Velella is wafted along by the action of the wind, and has probably been 

 often mistaken by navigators for the fabled cephalopodic Paper-sailor 

 {Argonauta). Prehensile and digestive tentacula depend from the 

 under surface of the disc, in the centre of which, both in Porpita and 

 Velella, is a short tube, with an orifice that probably relates to the 

 excretory and respiratory functions. 



The Acalephae propagate by spontaneous fission, by gemmation, 

 and by impregnated ova: the parthenogenetic modes are chiefly 

 limited to the young or larval state. In some species the gem- 

 mation is incomplete, and a chain of organically-connected individuals 

 results, as, e. g. in the Diphyes campanulifera and the Stephanomia 

 prolifera. Both these and the Physophoridce are also oviparous. 

 In the Diphyidce the generative individual commences as a bud or 

 process of the common tube, and, after great changes in its form, 

 becomes provided, like the parent individual, with a natatory organ 

 and with a sac composed of two membranes suspended from its 

 centre. The ova, or the spermatozoa, according to the sex, are 

 developed between the two membranes of the sac ; the individual 

 then becomes detached, and swims freely about. 



In the Norwegian seas there are little Medusa?, very similar to 

 those called by Eschcholtz Cytais, in which Sars observed tliat gem- 

 mation took place from the sides of the pendant stomach ; this organ 

 has four sides, and from each of these proceeds a little bud, and these 

 buds are observed in dijfferent grades of development : they are 

 successively detached as they attain the form of the parent in mi- 

 niature, and soon acquire their full size.* In May, 1837, Sars ob- 

 served a similar gemmation in the Thaiimantias multicirrata, a 

 (probably larval) Acalephe, one inch in diameter.f In the Sarsia 

 prolifera Forbes found the buds produced at the bases of the four mar- 

 ginal tentacles, and hanging from them in bunches. The degree of 

 development is not equal in all the four bunches, and in each case buds 

 are seen in very various stages of development, from embryo wart-like, 

 sproutings to miniature medusae. \ Will states, that in the Beroe 

 (^Eucharis) multicornis he saw the tubercles and lobes occasionally 



• CLI. p. 10. pi. IV. 7 -13. t lb. t CXLVII. p. 17. 



