INTRODUCTION. Cl 



two opposite faces, separated by the ciliary corona one 

 (the aboral), which is much the most voluminous, at the 

 opposite end from the mouth, the other (the oral) bearing 

 the buccal orifice at its centre, and capable of being so 

 covered as to constitute a vestibule (Woodcut, fig. xliii.). 

 From this primitive form all the other larval types are 

 easily derivable, according to Barrois; "so far as the 

 first portion of their development is concerned, there is 

 a perfect uniformity throughout the entire group"*. 

 Provisionally he regards the three divisions of the Ento- 

 procta, the Cyclostomata, and the Escharina^ (of Smitt) 

 as being three parallel modifications of an ideal primitive 

 form (Woodcut, fig. xliii.). The Escharine larva (which 



Fig. xliii. 



Ideal primitive form of the Polyzoan group. 



o. Mouth of the gastrvla. 8. Oral face. /. Aboral face. C. Corona. 

 st. Stomach, mi. Aboral meaoderm. 



makes the nearest approach to this ideal primitive type) 

 gives origin to two divergent and strongly marked modi- 

 fications (with which it is connected by transition forms) 

 that of the Cellular ina (Smitt) and that of Cyphonautes, 



Barrow, ' Embryologie,' &c., p. LSI . 



t The Exharina of Smitt include almost the whole of the I'licilostomatous 

 forms in which the rocecium has a solid calcareous front wall. His Crllu- 

 larina embrace nil tin- Cheilostomatous families which rank, in the present 

 work, before the Ctllanida 1 (p. 103). 



