TETRACTINELLIDA 



213 



consists of microcalthrops and their derivatives. The hypopliare 

 is well developed. Plakina thus shows a certain amount of 

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

 remarkable possession of flagellated pinacocytes. 



One of the species of Tetilla, T. pecUfera, continues the 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 Sijcon 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 com- 

 pressa from the north of Scot- 

 land and Orkney and Shetland 

 is at about the same stage of 

 development, 1)eing witliout Fig, 

 cortex and having eurypylous 

 chambers, but it is not so good 

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



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

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

 of homologous origin in tlie various cases, but probably to 



10."i. — Diagrniiimatic vertical sections 

 of A, Rhao-on ; B, Plakina : C, Tellllu 

 pedifera. 



fb 



IT 



B 



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

 dermal ostia ; fb, fibrous tissue ; i.e. iutercortical cavity ; sd, subderinal cavity ; 

 sji, sphincter. (After Sollas.) 



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

 CranieUa respectively (Fig. 106). 



