VIII 



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. 



T. pedifera, continues the series. 



f%i\ 



One of the species of Tetilla, 

 The folds of its choanosome are 

 more complicated than 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 Poecillastra com- 

 pressa from the north of Scot- 

 land and Orkney and Shetland 

 is at about the same stage of 

 development, being without Fig. 105 

 cortex and having eurypylous 

 chambers, but it is not so good 

 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 Choristida ; these possess a cortex not 

 of homologous origin in the various cases, but probably to 



Diagrammatic vertical sections 

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



pedifera. 



M 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). 



