REPORT ON THE SIPHONOPHORiE. 31 



in the Velellidae elliptical. In the expanded state the mouth may assume the most 

 different forms, trumpet-shaped, discoidal, &c. 



The thick wall of the central siphon is always very contractile, and composed of 

 five layers: — (l) An outer epithelium or epidermis, rich in cnidocysts ; (2) a very 

 thick layer of strong longitudinal muscles ; (3) a solid fulcrum or a homogenous 

 structureless elastic supporting plate ; (4) a thin layer of circular muscles ; (5) an 

 inner epithelium or entoderm. The mouth is armed with peculiar cnidocysts and pro- 

 vided with sensillas. The upper or proximal half of the central siphon contains in its 

 epidermis often (but not always) a number of bent tracheae, which end here. 



Gastrobasal Plate. — The basal or proximal part of the central siphon in all 

 Disconectae is separated from the superjacent centradenia by a strong fulcrum, or a 

 structureless elastic supporting plate (lamina gastrobasalis). This horizontal fulcral 

 plate (also called the roof of the central polypite, " le plancher " of Bedot, 59, 60) is 

 covered on the upper face by the basal surface of the centradenia, on the lower face 

 by the entodermal epithelium of the stomach. It corresponds to the jelly-plate which 

 forms the roof of the manubrium in the Medusae. Its central part is solid, whilst its 

 peripheral part is pierced by eight or more gastral ostia ; these form an octoradial corona 

 in the Discalidas and the smaller Porpitidae, whilst their number is increased in the 

 larger forms of the latter family (sixteen to thirty-two or more). Sometimes the 

 numerous ostia form here vertical lanceolate fissures, and the septa between them form 

 an elegant multiradiate star, composed of numerous vertical lamellas. The Velellidae 

 exhibit instead of this regular star a bilateral arrangement of the gastral ostia ; they 

 form here two opposite longitudinal rows of fissures (usually sixteen) on the two lateral 

 margins of the lanceolate gastrobasal plate. 



Centradenia or Central Gland. — The central space of the body, between the apical 

 or proximal pneumatophore and the basal or distal central siphon, is in all Disconectae 

 occupied by a peculiar large glandular organ, wanting in all the other Siphonophorae (or 

 Siphonanthse). This interposed central organ is usually called the liver (hepar); but as 

 its structure and function are complicated and not merely hepatic, it may be better 

 called centradenia, or central gland. It is composed essentially of a dense network of 

 entodermal gastral canals, and of a compact parenchyma of exodermal epithelium, with 

 innumerable cnidoblasts, filling up the meshes or intervals of that network. The 

 physiological function of the gastral canals may be partly hepatic (digestive), partly 

 renal (excretory) ; the exodermal epithelium, however, seems partly to perform the 

 function of a pneumadenia (or gas-producing gland), partly to be a large reservoir of 

 cnidoblasts for other purposes. 



The Form of the Centradenia is in general lenticular or discoidal, sometimes sub- 

 globular or even cylindrical, sometimes more conical or flatly expanded ; its peripheral 

 outline is circular, or sometimes regularly octagonal, in the Discalidae and Porpitidae ; it 



