PALUDICELLA EHRENBERGI. 115 



septum, visible through the transparent walls of the cell ; the entire ccencEcium thus presents 

 a very striking and elegant concatenated appearance. The orifices are unilateral, and are 

 placed at the extremity of little tubes, which project obliquely forward from the side of the 

 cell near its wide end. The branches are given off at nearly right angles from the wide part 

 of the cell, one at each side of the tulniiar orifice, and are thus (unless when the suppression 

 of one of them takes place) situated in pairs one opposite to the other, with the tubular 

 orifice between them. 



The ectocyst is a pellucid membrano-corneous tunic, nearly colourless in young specimens, 

 but acquiring a dark brown colour by age. 



The polypide has but sixteen tentacula, springing from the margin of the orbicular 

 lophophore, in a perfectly infundibulate crown. 



Van Beneden* describes the occurrence of what he terms " hybernacula," in PahuVceUa 

 Ehrenberr/i. These hybernacula are gemmce, which, under the influence of a fovorable 

 temperature, would have grown into the ordinary lateral branches of the Polyzoon, but which 

 towards the commencement of winter acquire a conical form, and then become for a while 

 arrested in their development. In this state, surrounded by a firm membrane of a blackish- 

 gray colour, they continue until the following spring, when the investing membrane splits to 

 allow of the elongation of the branch. In no instance have I witnessed the hybernacula in 

 the specimens of Paludicella collected in Britain. 



P. EJirenbergi is an exceedingly timid little animal, and a specimen may be for hours 

 under observation before the polypides will venture to issue from their cells, and then it is often 

 for only a few seconds at a time, that they will continue visible. It is eminently a lover of 

 obscurity, being only found in such situations as are not exposed to the direct influence of 

 daylight; the most luxuriant specimens I have met occurred under the arches of viaducts on 

 the Grand Canal, near Dublin ; and in similar situations elsewhere, where the constant 

 obscurity in which they live is never interrupted by a single ray of direct daylight. 



The species occur all through the year in still and slowly running water, but is most 

 abundant during the summer and autumn. 



After a careful consideration of the animal described by Mr. Hancockf under the name 

 of P. procumbens, I have been unable to agree with this naturalist in considering it a distinct 

 species. The main character on which Mr. Hancock relies, is the supposed smaller number 

 of tentacula in his species, a belief to which he was naturally lead in consequence of the 

 figure of P. Ehrenberiji given by me in the 'Proceedings of the Royal Irish Academy, 1843/ 

 having been erroneously engraved with twenty-si.x tentacula. Mr. Hancock also relies on the 

 less densely and luxuriantly branched condition of his specimens, and on a smaller size and 

 difference of colour in the cells, as grounds for distinction, but I believe these to be characters 



* Dumortier and Van Beneden, ' Hist. Nat. des Pol. Com. d'eau douce, Mem. servant d"e 

 Complement au tome xvi des Mem. de I'Acad. Hoy. Bruxelles.' 

 f Hancock, in 'Ann. Nat. Hist.,' 2d series, vol. v, p. 201. 



