llErA-TTONS or U AIiDOPLEURA. 



585 



complete, there is reason to believe that it is formed, like the 

 lojihophore and /tentacles, by an introversion of the polypide walls 

 ill the region of the mouth, and that it has thus a significance 

 entirely different from that of the shield of BliaMopleura. 



The formation of tlie chitinous ectocyst offers another question 

 by no means easy of solution. We know that in the ordinary 

 Polyzoa the ectocyst is a simple excretion from the surface of the 

 endocyst, which is continually in contact with it. In Rhabdo- 

 pleura, during the early stages of development of the polypide, 

 the cord which represents the chief part of the endocyst is much 

 thicker than at a later period, and may have then allowed the 

 chitinous tube to be moulded on its surface. I am, however, 

 well inclined to believe that the function of excreting the ecto- 

 cyst devolves on the shield, which at an early period is relatively 

 very large. It possesses, too, at this period a structure which 

 might quite accord with such a function, being composed of elon- 

 gated prismatic cells whose ends abut upon its outer surface. 

 Indeed we can hardly avoid comparing it in this function, as well 

 as in its form, with the shell-secreting mantle of a Lamellibran- 

 chiate mollusk. 



If we bring together the morphological facts here adduced, we 

 shall find that they give us a series which, so far as it goes, re- 

 presents the life-history of Wiahdopleura. We have the endocyst. 

 which, notwithstanding its anomalous condition, retains its normal 

 faculty of originating new zooids by gemmation. In Bhahdo- 

 pleura, however, the direct product of this faculty is a shield- 

 like zooid, which by its bivalve form in B. Normani may even 

 suggest the Cyphonautus-^i^ge, of 3femhranipora ; and it is from 

 this that we find emitted the ultimate bud which becomes directly 

 developed into the proper polypide. The developmental pheno- 

 mena here differ from those iu Alcyonella mainly by the interca- 

 lation of a scutiform zooid between the cystid and the polypid. 

 This zooid does not perish after the completion of the polypid, 

 but remains as a subordinate appendage of the latter. 



We are yet entirely ignorant of the sexual reproduction of 

 Rhahdnpleura ; and until this is discovered our knowledge of its 

 life-history must continue incomplete. 



It must be now evident that whatever apparent resemblance 

 there may be between Rhahdopleura and the proper Phylactolse- 

 mata, this genus essentially differs not only from the Phylacto- 

 Isemata, but from all other Polyzoa to such an extent that it will 



LINX. JOURN. — ZOOLOGY, VOL. XIY. 43 



