396 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



Hence, it has been pointed out that the characteristic poly- 

 pide of the ectoproctous Polyzoa is a structure developed 

 from the cystid, in much the same way as the Tcenia-he&d is 

 developed from its saccular embryo; or as the Cercaria is de- 

 veloped from the sporocyst, or Media ; the cystid of the Phy- 

 lactoloemata being comparable to a sporocyst, and that of Mem- 

 branipora to a Redia. But, without altogether denying the 

 justice of this comparison, it may be suggested that the cys- 

 tid is comparable to a vesicular morula, and that the mode of 

 development of the alimentary canal of the polypide corre- 

 sponds with that of the formation of an alimentary sac by in- 

 vagination. If this view of the case be correct, the perivisce- 

 ral cavitjr in the Polyzoa is a blastoccele, more or less modified 

 by the development of the mesoderm. 



The only known representative of the genus Rhabdopleu- 

 ra 1 is an aberrant Polyzoon which presents many interesting 

 peculiarities. The polyzoarium consists of a creeping stem 

 from which erect branches, each of which ends in a circular 

 aperture and constitutes the cell of a polypide, arise. The 

 cavity of the stem is divided by transverse septa, and its 

 centre is traversed by a hollow chitinous cord, which passes 

 through and is attached to the septa. 



The lophophore resembles that of the hippocrepian Phy- 

 lactolwmata in being produced into two arms, fringed with a 

 double series of tentacula. These arms are longer, narrower, 

 and more cylindrical than in any other Polyzoa, and, thus far, 

 approach the arms of the Prachiopoda. Furthermore, the 

 tentacula are confined to the arms, which are very flexible. 

 Betweeen the bases of the arms there is a rounded or pen- 

 tagonal disk with raised and ciliated edges, which occupies 

 the place of the epistoma in the phylactolsematous Polyzoa. 

 The mouth is situated beneath the free margin of this disk, 

 on the opposite side to the anus, and to that toward which 

 the arms are turned. The animal is attached to the bottom 

 of its cell, or rather to the endosarc of the stem, by means of 

 a long contractile pedicle, by which its retraction is effected. 

 According to Sars it protrudes itself by climbing up the wall 

 of its tubular cell by means of the disk. Prof. Lankester's 

 comparison of the polypide of Rhabdopleura to the embryo 

 Plsidium* appears to me to be fully justified. The branchiae 

 of Nucula, in form and position, present no little resemblance 



1 See the papers of Allman and G. 0. Sars, Quarterly Journal of Micro- 

 scopical Science, 1869 and 1874. 



2" On the Developmental History of the Mollusca." ("Phil. Trans.," 1874.) 



