SEA-URCHIXS. 



459 



warrant a belief that ariibulacral brancliia! were present. And 

 so in Conidus, in tlie almost complete absence of peiistomial gills, 

 there seem to have been no special structures, either left or 

 developed, to perform the function of respiration. In the 

 Echinoneida3 the same condition obtains, but the Olypeastroida 

 show an ever increasing pei'fectiou of adnpical petals to com- 

 [)ensate for the loss of the moi'e primitive adoral branchiaj. 



3. The Perignathic Girdle. 



Detailed and valuable studies of this structure in Discoidea and 

 Gottidus have been made by Duncan and Sladen (40 cfe 41) and 

 Loven (43 & 48). These researches have the additional value 

 that they were pursued with different aims. The former authors 

 wei'e intent upon demonstrating the absence of jaws in the two 

 genera, while Loven predicted, and later realised, their discovery 

 in Discoidea. The structure of the perignathic girdles of Pygaster 

 and Ilolectypus is not so fully known, and in the case of 

 AnortJiopygus thei'e wei-e no known ti-aces of the girdle when 

 Duncan (44) placed the genus in the same family with Kchino- 

 ueus. Loven (48) knew of its existence in all the three genera, 

 but gave no details of its structure. 



Text-fig. 55. 



C D 



Diagrams of the perifiimtliic girdles in 

 A. Pj/gasfer. B. Anorthopifgus. C Discoidea. D. Conidus. 



In Pygaster the processes are very strongly developed, while 

 the ridges are hardly recognizable. There is no tendency for the 

 processes to form an arch over the ambulacra — in fact, they 

 slope away from one another. Thus there is iriitiated the 

 persistently disjunct girdle which characterizes all the gnatho- 

 stomatous Irregular Echinoids. In Ilolectypus the structure 

 seems to have been practically the same, but there are indications 

 that the ridges were slightly more pronounced. This was 

 certainly the case in Anorthopi/giiK. In JJiscnidra the processes 



