442 COMPARATIVE ANATOMY CHAP. 



thus attains a large size. The membrane which completely separates 

 the perioesophageal sinus from the general body cavity then becomes 

 the lantern membrane, surrounding the masticatory apparatus on all 

 sides, from the point where the intestine leaves the lantern down to 

 the perignathous apophysial ring. The greater part of the periceso- 

 iihiigeal sinus (Fig. 358, 44, p. 419) is filled by the masticatory frame- 

 work, the remaining space being traversed by trabeculre, bands, etc. 

 All the radial organs arise from within the perioesophageal sinus, 

 running in it as far as to the auriculae. 



The outer gills and Stewart's organs of the Echinoids are develop- 

 ments from the perioesophageal sinus. 



The outer gills (Fig. 358, 1) consist of five pairs of branched 

 appendages, which rise at the periphery of the oral region, at the 

 inner edge of the oral integument, and project freely outward. One 

 pair of such gills occurs on each interradiu.s. The peristomal edge of 

 the disc is indented at certain points for the reception of the gills, so 

 that their presence or absence may be determined by the presence 

 or absence of these incisions on the test of either extant or fossil 

 Echinoids. The gills are hollow outgrowths of the oral integument, 

 their cavities being direct prolongations of the perioesophageal sinus, 

 and thus in open communication with this latter ; the body fluid of 

 the sinus can thus enter the gills and flow back into the sinus again. 

 The wall of such a gill consists of a deep outer epithelium provided 

 with long cilia, a central layer of connective tissue with calcareous 

 corpuscles and lacunae, and an inner ciliated covering of endothelium. 



Kxternal gills occur in most endocyclic (regular) Echinoids. They 

 seem to be wanting only in the (.'i(/>n-/itn. 



Stewart's organs. Just as external bulgings of the oral integu- 

 ment lead to the formation of external gills, internal bulgings of the 

 lantern membrane into the body cavity give rise to Stewart's organs. 

 These are delicate-skinned vesicles or tubes, which vary greatly in size. 

 Five are usually present, projecting into the body cavity from the 

 edge of the (apically directed) base of the masticatory framework, 

 immediately below the fork plates or radii between these and the 

 falces or intermediate plates (cf. p. 400). The cavity of the vesicle 

 is a diverticulum of the perioesophageal sinus. 



The Stewart's organs of the Ciclaroida are large, and are usually beset with 

 secondary outgrowths. Since the Cidaroida possess no gills, it has been conjectured 

 that Stewart's organs fulfil the functions of the absent external gills, and they have 

 therefore been called internal gills. It is, however, very difficult to demonstrate 

 with any certainty the respiratory significance of these organs. 



In the Echinothuridcc, although external gills are present, Stewart's organs may 

 attain a gigantic size (Astlieiwsoma urens, Fig. 370). They here fill the greater 

 part of the body cavity, and, it has been conjectured, serve in this case to prevent the 

 collapse of the flexible test at the time when the genital products are ejected. On 

 the other hand they may be quite vestigial, or even altogether absent (Phormoeomd). ' 



1 Cf. Bell, Ann. Man. *' H. vol. iv. 1889, p. 437. 



