REPORT ON THE SIPHONOPHOPwE. 55 



fore finish in the solid glandular parenchyma of this latter, and cannot pass through it 

 and enter into the wall of the siphons. A small part only of the tracheae enters into this 

 latter. The distal ends of all tracheae are open, surrounded by exodermal epithelia, 

 as in the other Disconectae. No tracheae are found in the marginal tentacles, nor in the 

 peripheral part of the umbrella outside the centradenia. 



The margin of the pneumatoeyst is circular and not lobate in the flat discoidal Porpi- 

 tellidae, whilst it is divided into numerous radial lobes in the campanulate Porpalidae ; the 

 lobes are here sometimes small, sometimes widely prominent. Their number is originally 

 eight or sixteen, and by furcation thirty-two or sixty-four (PI. XLVIII. figs. 4, 5). 



Central Siphon. — The large central polypite of the Porpitidos is not different from 

 that of the Discalidae, a thick-walled and very contractile tube of very variable form. 

 Usually it is inversely conical, its diameter decreasing gradually from the broad proximal 

 base towards the distal mouth. Sometimes the upper half, or the stomach, is ovate and 

 much wider than the lower half, or the cylindrical proboscis. Its transverse section is 

 either circular or octagonal, in consequence of eight prominent radial folds. In some larger 

 species the outer wall exhibits sixteen longitudinal folds instead of eight, and sometimes 

 eight larger (perradial) and eight smaller (interradial) ribs alternate. To these correspond 

 the same number of internal furrows at the inside of the siphon. These furrows lead into 

 the basal openings of the stomach, in which the primary radial canals open (eight or 

 sixteen). The basal ostia form a regular corona ; in some larger species their number is 

 increased, numerous secondary and tertiary ostia being intercalated between the eight 

 primary ones. The base of the stomach is separated completely from the overlaying 

 centradenia by the structureless sobd gastrobasal plate. 



Centradenia. — The large central gland, or the so-called " central organ" (formerly 

 "fiver"), exhibits the peculiar composition described above (p. 31). In the Porpitidae 

 it is much more voluminous than in the Discalidae and Velellidas, and occupies the whole 

 space between the inferior face of the pneumatophore and the superior face of the sub- 

 umbrella which bears the siphons. The central gland is largest in some lenticular or 

 subglobular Porpalidae, where its weight and volume are greater than those of all other 

 parts of the body together ; it is relatively smaller in the flat discoidal Porpitellidae. 

 The dense network of canals in the central gland is in its upper brown or blackish half 

 composed of hepatic vessels (with biliary epithelium), in its lower green or whitish half of 

 renal vessels (with epithelium secreting guanin crystals). The compact exodermal epithe- 

 lium filling up the intervals of the canal-network contains masses of cnidoblasts and 

 probably secretes the gas, which enters into the open distal ends of the tracheae. 



Gonostyles. — The numerous polypites of the subumbrella, which produce by budding 

 the medusiform gonophores, in the Porpitidae are not mouthless palpons as in the Dis- 

 calidae, but mouth-bearing siphons as in the Velellidae. They are, therefore, usually called 

 " smaller polyps, sexual polypites, or peripheral siphons" (shortly " perisiphons"). They 



