142 



COELEXTERATA. 



Ik 



small apertures the stigmata and communicates with the centra- 

 denia and adjacent tissues by a number of fine tubes the tracheae 

 which project from its base. The tracheae are lined by a prolongation 

 of the chitinous lining of the pneumatocyst. 



The centradenia is separated from the dorsal endoderm of the 

 stomach by a gelatinous plate the gastrobasal plate (sp) which is 

 pierced by the gastral ostia (primitively 8) for the passage dorsalwards 

 of the radial canals (?&), whicl^ arise from the fundus of the stomach. 

 It is a composite organ, partly consisting of a dense network of 



endodermal gastral 

 canals, and partly of 

 a parenchyma of 

 ectoderm cells with 

 many cnidoblasts. 

 The function of the 

 former is probably 

 , partly digestive and 

 p^s^% partly renal, of the 

 latter gas-producing. 

 In the simpler Dis- 

 conectae, i.e., in the 

 Discalidae the cen- 

 tradenia is composed 

 solely of a compact 

 mass of ectoderm 

 cells and cnidoblasts 

 without canals. The 

 only canals of the 

 centradenia of such 

 forms are eight sim- 

 ple radial canals 



which arise from the eight ostia of the stomacli and run on its 

 upper face between it and the pneumatosac uniting in its centre 

 to form a typical octoradial liver star. They are to be looked upon 

 as ascending branches of the eight' primary radial canals of -the sub- 

 umbrella. The canals which perforate the centradenia of the more 

 complex forms are branches of these (Fig. 117). 



The canal system is primitively octoradiate; it consists of the 

 radial canals above mentioned, which branch as they pass outwards 

 to be united at the margin of the umbrella by a circular canal (not 

 marked in the figure). This system gives off, in addition to the 



FIG. 117. Porpalia prunella (after Haeckel from Lang), cd 

 centradenia (central gland); Ik pneumatocyst; cp central 

 stigma of pneumatocyst ; rlc radial canal ; sp gastrobasal 

 plate; eii ex-umbrella; su sub - umbrella ; t tentacles; g 

 gonophores ; o mouth ; ms chief siphon ; gm accessory 

 siphons, in this case gonostyles. 



