396 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



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

 pide of the ectoproctous Folyzoa is a structure developed 

 from the cystid, in much the same way as the 7Ve?n'«-head is 

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

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

 lactolmmata being comparal3le to a sporocyst, and that of 3Iem- 

 hranipora to a Hedia. But, without altogether denying the 

 justice of this compaiison, 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. Jf this view of the case be correct, the perivisce- 

 ral cavity in the Poly zoa is a blastocoele, more or less modified 

 by the development of the mesoderm. 



The only known representative of the genus Rhdbdopleu- 

 ra ' is an aberrant Polyzoon which presents many interesting 

 peculiarities. The polyzoarium consists of a creeping stem 

 from w4jich 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 sei^ta, 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- 

 lactolcemata in being produced into two arms, fringed w^ith 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 Brachiopoda. 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 phylactolaematous 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 tlie polypide of Phahdopleura to the embryo 

 Plsidium^ appears to me to be fully justified. The branchi?^ 

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



1 See tlie papers of Allniau and G. 0. Sars, Quarterly Journal of Micro- 

 icopical Science, 1869 ami 1874. 



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



