Notes on some Hj'dromedusae from tlie Bay of Naples. 559 



mecbanieal process due, in part at least, to a reductiou of the 

 eutodermie cellmass wliich has been involved in the transformation 

 of a considerable portiou of these epithelial cells into purely nutri- 

 tive cells concerned iu the growth of the eggs. Abundant evidence 

 of this is fouud in the manifest signs of degenerative metabolism 

 occurring' throughout the entire mass. In many cases the cellular 

 character had been lost entirely, in others it had been greatly modi- 

 fìed. Figs. 4 and 7 show something of these changes. 



With these phases of development completed the medusoid fea- 

 tures may be said to l)e well established, though velum, tentacles, 

 and canal Systems are lacking. Only the first of these organs is 

 iinally developed. Tentacles are wholly absent; the bud-like pro- 

 jectious shown at ni in Fig. 7 are but sectional aspects of the beli 

 margin, which as is shown in Fig. 2 of the fully developed medusa, 

 is thiek and rim-like. The velum is formed shortly before the birth 

 of the medusa by a double fold of ectoderm, as shown in Fig. 7 v. 

 This fold is apparently continuous over the entire velar end of the 

 medusa, until shortly before birth, when a circular openiug is formed 

 by what seems to be an absorption of the centrai cells. 



Of the canal system there is little to be said, since it is at 

 best quite rudimentary. In Fig. 7 ce is shown what in general po- 

 sition and form may be considered as the marginai canal. In no 

 case have I been able to find traces of radiai canals. In certain 

 specimens sections would show a deeper fissiou of the entoderm 

 layers at the junction of the apical projection and the lateral walls 

 of the umbrella, and in others an extension of the marginai canal 

 upwards, but there was no communication between the two. Further- 

 more, the manubrium is permanently mouthless, so that the medusa 

 is apparently doomed to an ephemeral existence as au iudependent 

 organism. And such is indeed the case. Of many specimens libe- 

 rated from hydroid colonies in the aquarium none lived for more 

 than an hour or two at most, as indicated in an earlier connection. 



The apical projection is bulb-like, very thin and hollow as shown 

 iu Fig. 7. It is lined internally by an extremely delicate and 

 tenuous layer of entoderm, the presence of which only becomes de- 

 monstrable under the highest powers of the microscope. The ecto- 

 derm layer over this region is likewise very thin, though more 

 easily distinguished than the former. 



As the medusae approach the period of liberation numerous 

 orauge-colored pigment granules ai)i)ear in and among the entoderm 



1 37* 



