REPORT ON THE SIPHONOPHORiE. 43 



accessory stigmata, lying in one or in several of the concentric ring-chambers. Usually 

 the third or fourth only of these exhibit eight stigmata, placed in the interradial lines, 

 which pass through the former. But sometimes these accessory stigmata are irregularly 

 scattered. The gas enclosed in the pneumatocyst may issue by these stigmata, when the 

 strong muscle-plate of the surrounding pueumatosaccus contracts. 



Tracheal (figs. 5, §,pt). — The inferior (basal or distal) face of the pneumatocyst bears 

 the aeriferous tubules which receive the gas secreted by the pneumadenia and conduct it 

 into the chambers of the former. The simplest genus, Disccdia (fig. 5), possesses only 

 eight short tracheae, which arise from the peripheral part of the inferior side of the eight 

 triangular radial chambers. The more highly developed Disconalia (fig. 9) exhibits, 

 besides these latter, a greater number of peripheral tracheae, arising from the lower face 

 of the concentric ring-chambers ; they are more numerous in the innermost than in the 

 middle ring-chambers, and are wanting in the outermost. Their number amounts to from 

 twenty to eighty or more ; their arrangement is variable and irregular. The tracheae in 

 all Discalidse are very short and small, their cylindrical articulate tubules composed of 

 ten to thirty small conical segments. They are more or less irregularly curved, and 

 descend in various directions into the solid glandular parenchyma of the centradenia, 

 where their open distal ends are surrounded by exodermal cells. In no Discalidae do the 

 short tracheae pierce the entire centradenia and the subjacent gastrobasal plate, nor do 

 they enter into the base of the wall of the central siphon and the gonostyles, as is the 

 case in the Porpitidae. 



Central Siphon. — The large central polypite, which corresponds to the manubrium or 

 gastral tube of the original Medusa, in the Discalidae is relatively larger and more 

 developed than in the Porpitidae and Velellidae. It is in the former the only organ for 

 the reception of food and digestion, whilst these nutritive functions in the two latter 

 families are executed also by the sexual peripheral siphons. The central siphon of the 

 pyriform Discalia (PI. XLIX. figs. 1-4, so) is very elongated, and about as long as the 

 greatest diameter of the umbrella, whilst it is much smaller in the discoidal Disconalia 

 (PL L. fig. 1) ; its diameter (in length and breadth) is here only one-third or one-fourth 

 of the latter. The basal part (or stomach) is ovate or pyriform, the distal half (or 

 proboscis) cylindrical. The thick, very contractile wall is composed, as usual, of a 

 stronger exodermal longitudinal layer of muscles, and a thinner entodermal layer of 

 circular muscles, separated by an elastic structureless support. 



The fundus of the stomach is separated from the superjacent centradenia by the 

 horizontal gastrobasal plate. The periphery of this solid circular or octagonal support 

 exhibits eight equidistant openings, the ostia, which conduct into the eight radial canals 

 of the subumbrella. These ostia are prolonged sometimes downwards into eight longi- 

 tudinal grooves at the inside of the stomach, and to these correspond eight longitudinal 

 folds or ribs on its outside. 



