POLYZOA. 



6ll 



other cases, a projecting tubule. Mr Waters has further shown that 

 in the Cyclostomata it is not uncommon for a calcareous plate to be 

 developed at the point where the zocecial tube rises free from the 

 polyzoarium ; while in other cases, two of these " closing-plates " 

 are developed close together. Probably of a similar nature to the 

 " closing-plates " just mentioned are the transverse calcareous parti- 

 tions which are developed in the tubes of various Cyclostomatous 

 Po/yzoa, both recent and fossil, and which have been spoken of as 

 " tabulae," from their similarity to the structures known by this name 

 in many Corals. These so-called " tabulae " differ from the " clos- 

 ing-plates " described by Mr Waters in being developed in the deeper 



Fig. 456. — Minute structure of the recent Heteropora pelliculata, Waters ( = //. neozelanica, 

 Busk), a. Tangential section of the polyzoary, enlarged about fifty times, showing the proper 

 zooecia (a, a) and interstitial tubes (b) connected by numerous minute tubules (c). The cells 

 are furnished with numerous radiating spines or "rays." b, Part of a longitudinal section, 

 enlarged about twenty times, showing connecting-pores, "rays," and delicate transverse parti- 

 tions or " tabulae." (Original.) 



as well as in the more superficial portions of the colony, and they 

 are sometimes present in considerable numbers. They occur in 

 various genera, as, for example, in Hete?-opora (fig. 456, b), Alveo- 

 hiria. Fascicularia, Lkhe?iopo?-a, &c. : and they are probably con- 

 nected with the periodic death and renovation of the polypides 

 which is known to occur in many recent Polyzoa. 



In certain of the Polyzoa, finally, the cells are furnished with 

 delicate, radially disposed spines, which present a superficial re- 

 semblance to the " septa " of such corals as Favosites and certain 

 of the Heliolitidce, and which may be spoken of as " rays." These 

 curious structures (fig. 456, a) have the form of minute calcareous 



