6l2 MOLLUSCOIDEA. 



spines which are directed into the interior of the cell-cavity, their 

 free ends being sometimes globular or button-like. They have 

 been recognised by the present writer in the recent Heteropora 

 pelliculata and in some species of Lichenopora (Discoporella) ; while 

 Mr Waters has detected them in Entalophora ijitricaria, and in species 

 of Tabulipora and Lichenopora ; and apparently identical structures 

 have been shown by Mr John Young to exist in certain of the Fenes- 

 tellidcB. So far as known, these "rays" are confined to certain of 

 the Cyclostomatous Polyzoa, and their precise morphological signifi- 

 cance has not as yet been determined. According to a curious 

 observation of Mr Waters, the zooecia of Lichenopora grignonensis 

 not only possess " rays " in their interior, but are also furnished 

 with similar spines projecting from portions of the external surface. 



As regards their classification, the Polyzoa may be divided into 

 the three primary sections of the Ectoprocta, Entoprocta, and Aspido- 

 phora, of which the last two require no further characterisation here, 

 as they are not known to possess any fossil representatives. The 

 division of the Ectoprocta, on the other hand, includes the great 

 majority of the living Polyzoa and the whole of the fossil forms, 

 though the essential feature of the division is the position of the 

 anal opening outside the tentacular circle, — a character which, 

 necessarily, can only be positively determined in living examples. 

 The Ectoprocta may be divided into the two orders of the Phylacto- 

 Icemata and Gymnolcemata, the former characterised by the almost 

 universal feature that the tentacular crown is horse-shoe-shaped, and 

 by the possession of a peculiar valve-like organ ( t: epistome ") arch- 

 ing over the mouth ; while the tentacular crown in the latter is 

 circular and there is no epistome. The Phylactolaematous Polyzoa 

 are confined to fresh waters, and they need not be considered here 

 further, as no fossil examples of the order have ever been detected. 

 The order of the GymnolcEmata, on the other hand, is of great im- 

 portance, as comprising the vast majority of the existing marine 

 Polyzoa and all the known fossil forms. The order may be divided 

 into the three sub-orders of the Cyclostomata, the Cheilostomata, 

 and the Ctenostomata, the first two characterised by the form of 

 the cell and the absence or presence of an opercular valve to the 

 cell-aperture, while the last is distinguished by the fact that the 

 cells arise from a common tube, their mouths being terminal and 

 furnished with a setose fringe for their closure. 



As regards their distribution in time, the earliest known forms of 

 the Polyzoa are found in the Ordovician rocks, and all the later 

 Palaeozoic formations abound in Polyzoan remains. Even in the 

 Ordovician rocks the Gymnolceniata are represented by numerous 

 and very widely different types, and the primitive forms of the class 

 must have existed in Cambrian or in even earlier periods. The 



