PORPITA. 329 



staining cells of irregular outline (Plate 28, fig. 15). In none of the specimens 

 sectioned have I been able to find the guanin crystals so characteristic of the 

 "kidney" region in Porpita. In this respect Porpema resembles Velella, as 

 indeed it does in the general simplicity of the centradenia. 



Color. Porpema prunella is a very beautiful object when floating on the 

 water, the central disc being deep ultramarine blue, most intense at the centre, 

 on which the radiating ridges are marked by pale lines. WTien seen from the 

 side, the pneumatophore shines through the nearly transparent zone composed 

 of the bases of the tentacles as a deep blue central core ; the general mass of the 

 centradenia below it is a pale reddish purple. The gonozooids are pale blue, 

 their medusae yellowish; tentacles bluish at the base, but soon becoming pale 

 yellow, their nematocyst knobs violet. 



PORPITA Lamarck, 1801. 



It has recently been suggested that the name Medusa, instead of Porpita, 

 should be employed for the genus now under consideration. I may therefore 

 summarize the grounds upon which I retain the latter. The first appearance 

 in binomial literature of an animal referable to this genus is in the 10th edition 

 of the Systema Naturae, where, under the name Medusa porpita, Linne briefly 

 diagnoses a form earlier (1754) described by him under the name "Medusa 

 (Porpita) orbicularis" and which from his earher figures (1754) is undoubtedly 

 founded on a fragmentary Porpita. In 1776 O. F. Miiller described the Atlantic 

 Porpita under the name Medusa uvibeUa. Forskal (1775, 1776), however, who 

 gave an excellent account of the Mediterranean form now generally known as 

 P. umbella, referred it to the Holothuria denudata of Browne, although, as Chun 

 ('97b) has recently pointed out, the latter name evidently belonged to a frag- 

 mentary Salpa. 



Gmelin (1790) retained Forskal's species in the genus Holothuria, as H. nuda, 

 distinguishing, and rightly, between it and Browne's H. denudata, but he doubted 

 whether H. nuda really belonged to Medusa umbella. Almost simultaneously 

 Modeer (1790) used for Porpita the name Phyllodice, apparently without realiz- 

 ing that the animal (Velella) to which Browne (1789) had already applied that 

 name was generically distinct. The genus Porpita was instituted in 1801 bj- 

 Lamarck, for the Medusa porpita of Linne. Since that date it has been usuallj' 

 accepted. The only grounds on which the name Medusa could be substituted 

 for Porpita would be that the species porpita, of Linne, was the type of the 

 genus Medusa. 



