RHABDOPLEURA MIRABILIS. 27 



zoarium in the Rhabdopleura mirabilis is decidedly different 

 from that of the Shetland R. Normanni. Firstly, the 

 creeping stem of the latter, of which one surface everywhere 

 adheres to old shells or other solid sul)stances, is much more 

 strongly branched (" subalteruately "), and, like the cells, 

 quite naked, without trace of attached extraneous particles. 

 Then the cells themselves are not, as in our species, free in 

 their whole length, but at the base for some distance, like the 

 stem itself, fixed and creeping, for which reason also the free 

 perpendicularly rising ringed part of the same is much 

 shorter than in the Rh. mirabilis. Finally, there appears 

 some difference between the two species with regard to the 

 manner of the division of the stem into chambers. 



The polyzoarium in the Rhabdopleura does not coincide 

 with any other species of Polyzoa. The cells in their tubular 

 form resemble most of those of the Cyclostomata, but are 

 horny or chitine-like (in the Cyclostomata they are chalky), 

 and are distinguished by their surface, covered with promi- 

 nent transverse folds or rings, which are also foreign to the 

 Polyzoa, but are found in many Hydrozoa, such as certain 

 Tubulariadse and Campanulariadae. 



The individual animals or polypides seem at first glance to 

 resemble the ordinary Polyzoa (see fig. 5). The body, which 

 is only a little over 1 mm. long, is oblong, and appears to be 

 occupied almost entirely by the digestive system ; on closer 

 examination, we find, however (see figs. 1, 2, and 4), that 

 a thin glassy skin surrounds the digestive apparatus, 

 which therefore is not, as in all other Polyzoa, freely sus- 

 pended in the " perigastric fluid," which latter, as will 

 appear in the sequel, is entirely wanting in the Rhabdo- 

 pleura. In all other Polyzoa, without exception, there is 

 besides the so-called ectocyst, corresponding with the Poly- 

 zoarium, also a so-called endocyst, which always represents 

 a thin membrane, lining the interior of the ectocyst or cavity 

 of the cell, and, from the aperture, recurved and attached 

 round about to the polypide, under the base of the lopho- 

 phore. The interior cavity of the cells in the other Polyzoa 

 is thus actually completely closed by the endocyst and the 

 body of the polypide ; and the so-called perigastric fluid 

 therein contained, wherein the intestinal canal of the animal 

 is freely suspended, does not stand in any direct connexion 

 with the surrounding medium. The retraction of the poly- 

 pide into the cell is effected only by a folding (invagination)* 

 of the anterior elastic part of the endocyst, by which the 

 so-called tentacular sheath is produced. The case is quite 

 different with the Rhabdopleura. Here is no endocyst at all 



