TETRACTINELLIDA 



213 



consists of microcalthrops and their derivatives. The hypophare 

 is well developed. Plakina thus shows a certain amount of 

 resemblance to Oscarella (p. 196), with which it shares the very 

 remarkable possession of flagellated pinacocytes. 



One of the species of Tetilla, T. pedifera, continues tl>e series. 

 The folds of its choanosome are 

 more complicated tlian in P. 

 monolopha, and their outer ends 

 are bridged together by a thin 

 layer of ectosome (cf. species 

 of Sycon among Calcarea) ; the 

 chambers are still eurypylous. 



The skeleton reaches a high 

 level : it includes oxeas and 

 triaenes radiately disposed and 

 microscleres (sigmata) scattered 

 throughout the dermal layer. 

 The British Foecillastra coin- 

 pressa from the north of Scot- 

 land and Orkney and Shetland 

 is at about the same stage of 

 development, being without 

 cortex and having eurypylous 

 chambers, but it is not so good 



Fig. 105. — Diagrammatic vertical sections 

 of A, Rhagon ; B, Plakina : C, Tetilla 

 pedifera. 



an example, as the folds of its choanosome are confused. 



From T. pedifera we pass to the other species of Tetilla and 

 all the higher genera of Ghoristida ; these possess a cortex not 

 of homologous origin in the various cases, but probably to 



sp 



B 



Fig. 106 —A, Craniella type ; B, Stellettid type, ch, Chone ; co, collenchyma ; d.o, 

 dermal ostia ; fb, fibrous tissue ; i.c, intercortical cavity ; sd, subdermal cavity ; 

 sp, sphincter. (After Sollas.) 



be classified under one of two heads, typified by Stelletta and 

 Craniella respectively (Fig. 106). 



