66 THE VOYAGE OF H.M.S. CHALLENGER. 



granules " ; whilst another part is colourless or more or less filled up with guanin- 

 crystals ("renal granules"). The uppermost canals (between the pouches of the 

 pneumatocyst) are colourless. 



Central Sijjhon (figs. 1, 2, 3, sa).— The large sterile central polypite has the usual 

 form of a thick-walled cylindrical or inversely conical tube. Its dilated base occupies 

 the centre of the subumbrella, and is separated from the centradenia by a thick fulcrum 

 or supporting lamella, the gastrobasal plate ; it exhibits a peripheral ring of eight openings, 

 leading into the eight radial main vessels (compare p. 31). 



Sexual Siphons (fig. 2, fig. 3, sx).— The sixteen peripheral polypites form a regular 

 corona around the base of the large central polypite, and are separated by a circular furrow 

 from the corona of tentacles. They bear numerous gonophores on their thinner basal 

 part, whilst their spindle-shaped distal part opens by a four-lobed mouth (compare p. 37). 



Tentacles (figs. 1-3, t). — The numerous tentacles form a submarginal corona, which 

 occupies about one-third of the subumbrella. They form six to eight concentric rows, 

 and exhibit a very different length. Seen from above or below the corona presents 

 sixteen elegant rays, each of which is composed of an adradial bunch of larger tentacles. 

 The longest tentacles (in the middle of each radial bunch) surpass the diameter of the 

 umbrella in the contracted spirit specimens, and may be much longer in the living 

 animal. The form and structure of the tentacles, with three rows of cnidospheres, is that 

 which is usual in all Porpitidae (compare pp. 38, 39). 



Genus 6. Porpita, 1 Lamarck, 1816. 



Porpita, Lmk., Hist. nat. anim. s. vert., t. ii. p. 483. 



Definition. — Porpitidae with a flat discoidal umbrella, including a circular discoidal 

 pneumatocyst without marginal lobes. Tentacles very numerous, equally disposed in 

 several concentric circles, not forming radial bunches. 



The genus Porpita possesses the same flat discoidal umbrella, and the same circular, 

 not radially lobate, pneumatophore, as the preceding genus Porpitella ; but it differs from 

 the latter in the ecpial distribution of the tentacles along the whole margin of the 

 umbrella. The tentacles are very numerous, and densely crowded in several parallel 

 circles (as in Porpema), and they are not grouped into radial lobes. 



The genus Porpita, hitherto regarded as the only representative of the famfly 

 Porpitidae, was founded in 1816 by Lamarck upon the first known species, which Forskal 

 had observed in the Mediterranean, and described very accurately as Holothuria denu- 

 data. 2 Eschscholtz, the founder of the class Siphonophoras, in 1829 placed Porpita in 

 his family Velellidae, and gave the following definition : 3 — " Corpus orbiculare, supra 



1 Porpita = Ring' of a buckle, iripirn. * 11, p. 103, Taf. 26, fig. L 1. 3 1, p. 176, Taf. 16. 



