REPORT ON THE SIPHONOPHOR^E. 39 



The next larval stage of the Porpitidae exhibits four stalked cnidospheres at the 

 distal end of each tentacle ; ' one of these is the primary terminal knob, whilst the three 

 others are lateral branches (PI. L. fig. 10). By multiplication of the latter in the 

 prolonged tentacle arise three longitudinal rows of stalked cnidospheres, one of which 

 is situated on the inferior or distal side of the tentacle, the other two opposite on its 

 two lateral sides. These three longitudinal rows of short lateral branches, each of which 

 bears a spherical cnidonode, are very characteristic of all Porpitidse ; each row bears six 

 to nine or more branches in the larger species, their size decreasing from the terminal 

 (oldest) branch towards the basal (youngest). The basal half of the tentacles is usually 

 simple, and bears no branches. 



The tentacles of the Velellidae are never branched ; they always remain simple 

 cylindrical filaments, and are relatively short and small. The cnidocysts are either 

 irregularly scattered on their surface, or form two opposite ribands on the two lateral sides. 



Ontogeny. — The individual development of the Disconectae is very incompletely 

 known, but seems always connected with a peculiar form of metagenesis. The first 

 generation is asexual, the complicated cormus above described producing at its sub- 

 umbrella numerous polypites or secondary manubria (mouthless palpons in the 

 Discalidse, mouth-bearing siphons in the Porpitida3 and Velellidae). From the gastral 

 wall of these secondary polypites (surrounding the sterile central siphon) there arise 

 numerous medusiform buds of the form Discomitra. These do not become mature 

 whilst sessile and attached to their parents, but are soon detached, and develop into free 

 Hydroroedusae, which produce ova and spermatozoa. Some advanced stages of this second 

 sexual generation are described by Gegenbaur as Chrysomitra, and possess eight or 

 sixteen radial canals ; but they have not hitherto been sufficiently examined in the 

 adult state. The origin and structure of the sexual organs of the Disconectae, ovaria as 

 well as spermaria, require a further accurate examination. 



It is very probable (though not observed) that from the fertilised egg of this second 

 generation arises a young Medusa with eight radial canals, and that this early produces 

 in the top of its hemispherical umbrella the pneumatocyst, at first a simple central 

 chamber (comparable to the simple pneumatocyst of the Siphonanthas) and subsequently 

 a corona of eight radial chambers. From this common larval stage probably arise two 

 different lines of individual development. The Discalidas, on one hand, remain regularly 

 octoradial, and develop eight marginal tentacles (with increasing number), and between 

 these and the central siphon eight or sixteen gonostyles, remaining mouthless palpons. 

 The Porpitkke retain the same regular octoradial type, but are further developed, and 

 their gonost)des, at first mouthless, acquire afterwards a distal mouth-opening and 

 metamorphose into secondary siphons. 



On the other hand, a different course is followed by the bilateral Velellidas. Here 



1 Compare A. Agassiz, 59, pi. ix. fig. 1. 



