Development and metamorphosis of the larva of .inalma vIcKons 241 



Metamorphosis may be said to begin at the lime when, the second gastro/ooid 

 being still smaller than the protozooid, and further cormidial buds having already 

 made their appearance on the ventral side of the nectostyle (Figs. 3. 4), the stem of the 

 future nectosome appears (Figs. 6, 7) as an elongation of that part of the oozooid 

 that lies dorsal to the nectostyle. The nectostyle carries the larval bracts, rather like 

 the tip of a man's umbrella with its bare ribs spread open, and also the cormidial 

 buds. It remains at what may be called the nodal point of minimum growth, uhcrc it 

 can be recognized at the side of the stem for some time, whilst the elongating stem of 

 the nectosome carries the budding zone of the nectophores upwards and away from 

 it in an aboral direction. 



The second gastrozooid by now has overtaken the protozooid in size, and the adult 

 type of tentilla on its tentacle can be seen (Fig. 8). 



The upper end of the pneumatophore may have grown upwards as much as 4 mm 

 above the nectostyle before there is a noticeable elongation of that part of the stem 

 (siphosome) lying between the second and third gastrozooids. By the time that the 

 nectosome, measured from tip of pneumatophore to nectostyle, is 5-5 mm in length, 

 the siphosome, measurea from nectostyle to base of first and second gastrozooids, 

 may be 2 mm in length (Fig. 9). 



An interesting point about the post-larva during this stage of metamorphosis is 

 that the second gastrozooid comes to exceed the protozooid in size, and to assume a 

 terminal position, leaving the smaller protozooid on the dorsal side. The protozooid 

 too has a much smaller basigaster, which is partially divided on the outer, dorsal side. 

 Its relatively small size can be seen in my frontispiece (1954, Fig. D. Gz. (rd)). At 

 that time I had not studied the earlier developmental stages, which I have since 

 collected in abundance, and confused the protozooid with the first definitive gastro- 

 zooid. 



This side tracking of the protozooid is still more pronounced in Nanomia hijuga, 

 where it becomes vestigial. The significance of this seems to be that it marks a new 

 line of evolutionary experiment, an escape from the old line of specialization. This 

 is all the more probable since the primitive type of tentacle found on the protozooid 

 is replaced by a much more highly evolved type in the new series of gastrozooids. 

 This " adult " trifid type of cnidosac demands more and bigger nemaiocysts. The 

 basigasters of the secondary series of gastrozooids which supply the nematocysts are 

 correspondingly better developed. 



REFERENCES 



Fewkes, J. W. (1881), Studies of the jelly-fishes of Narragansett Bay. Bull., Mus, Comp. Zooi, 

 Harvard Coll., 8 (8), \4l-\S2. , ^., ^ ^. ^ , 



Fewkes, J. W. (1885), Studies from the Newport Zoological Laboratory. XV. On the development ol 

 Agalma. Bull., Mus. Comp. ZooL, Harvard Coll., 11 (11), 239-275. , w c- • 



Garstang, W. (1946), The morphology and relations ot the Siphonophora. Quart. J. SUcr. Set., 



87 (2) 103—193 

 Haeckel, E. (1869), ZurEntwickelungsgeschichtederSiphonophoren. Natuurk. Wrh.Prov. Utrecht sch. 



Genoots, (I), 6, 1-120. . , .... j c u u -^ 



Metschnikoff, E. (1874), Studien uber die Entwickelung der Medusen und Siphonophorcn. Z 



ToTTON, a!k. (1954), Siphonophora of the Indian Ocean together with systematic and biological 

 notes from other oceans. Discovery Repts., 11, 1-162. 



