CYCLOSTOMATA. 615 



of the fossil Polyzoa, no attempt will be made to give a complete 

 review of the different families. Not only is the classification of 

 the fossil Polyzoa in an admittedly unsatisfactory state, but the 

 characters of many of the fossil types cannot be rendered intelli- 

 gible without the free use of illustrations, even where these are 

 thoroughly understood ; while there are many types the structure of 

 which is at present only imperfectly known. Nothing further will 

 therefore be attempted here than to shortly characterise some of 

 the more important and more widely distributed groups, and to 

 indicate the chief forms of these and their general geological range ; 

 all the fossil types here referred to being provisionally distributed 

 between the Cydostomata and the Cheilostomata. 



Sub-Order I. Cyclostomata. 



This sub-order, as has been previously seen, includes Gymno- 

 laematous Polyzoa, in which the cells are more or less tubular, the 

 cell-aperture being terminal, usually of the same diameter as the 

 tube itself, and not provided with a movable operculum for its 

 closure. In a large number of the Cydostomata, the tubes (fig. 

 451, a) are free for a larger or smaller portion of their length, so 

 that the surface of the colony is partly formed by the lateral walls 

 of the zooecia ; while in other types (fig. 456, b) the tubes are in 

 contact throughout their entire length, the cells thus opening at 

 right angles to the axis or surface of the colony, and the whole 

 exterior being thus occupied by the cell-apertures. This difference 

 probably expresses a genuine structural distinction, and upon it Mr 

 Waters has proposed to divide the Cydostomata into the two divi- 

 sions of the Parallelata, with partially free tubes, and the Rectangu- 

 lata, with the tubes in contact throughout. 



In most of the Cydostomata the calcareous walls of the cells are 

 pierced by smaller or larger pores, but in some cases the walls 

 appear to be completely imperforate. In many forms all the cells 

 are similar to one another, but in others (e.g., in Heteropora) the 

 colony consists of two sets of tubes, which are similar in internal 

 structure, but differ in point of size. In other forms referred to 

 this sub-order an interstitial vesicular tissue appears to be developed 

 between the proper zocecia. " Ovicells " are developed in certain 

 of the Cydostomata, but they have not been observed in others, and 

 they do not play such a conspicuous part as they do in the Cheilos- 

 tomata. In various forms of the Cydostomata, finally, radiating 

 spines or " rays " are developed in the cells, while the structures 

 previously alluded to as " closing-plates " and " tabulae " are not 

 uncommonly present. 



The Cydostomata are the most ancient group of the Polyzoa, 



