14 DERMAL SYSTEM. 



Lophopiis, also, at first sight, conveys tlie impression of being destitute of an ectocyst, and 

 having the place of this tunic supplied by a peculiar unorganized gelatinoid secretion, in which 

 the colony is enveloped (PI. II, figs. 2, 3). This gelatinous-looking investment is, however, 

 a true ectocyst ; it consists of a membranous tunic of great delicacy, apparently enclosing 

 a perfectly transparent and colourless fluid, probably in the meshes of a sort of areolar tissue. 

 I have not, however, succeeded in making out in it any distinct structure, but its membranous 

 nature becomes at once manifest when the animal has undergone partial desiccation, for then 

 the ectocyst is thrown into folds by losing a portion of the fluid which had been imprisoned in 

 it. Neither Trembley nor Baker takes any notice of this gelatinoid envelope. M. Dumorticr 

 mentions it, and represents it in his figure,* while M. Van Beneden believes it to be an acci- 

 dental investment acquired by the animal during confinement.! 



The ectocyst in Paludicella is formed of a smooth pergamentaceous semi-transparent 

 membrane, free from earthy deposit, and of a deep brown colour. Towards the orifice of the 

 cell it becomes thin and delicate, and is here strengthened by four longitudinal horny ribs 

 (PL X, fig. 3, b"). The part of the ectocyst to which the ribs are attached is carried inwards 

 during extreme retraction of the polypide. These ribs I look upon as the true homologue of 

 the sei(e which crown the cell in Bowerbanhia and other ctenostomatous Polyzoa ; if these setae 

 were reduced in number to four, and instead of being free were attached along their entire 

 length to the sides of the cell, they would at once be converted into the ribs of Pahidicella ; 

 the fact of the setae in the ctenostomatous Polyzoa being connected to one another by a 

 delicate membrane does not in the least invalidate the view here taken, and the circumstance 

 of their being detached from the sides of the cell in these Polyzoa will account for the different 

 mode in which they are withdrawn during retraction. 



In certain species of fresh-water Polyzoa, transverse septa exist between the cells. They 

 are formed both by the ectocyst and cndocyst. In Paludicella they acquire their maximum 

 in development and constancy ; they occur here between every cell, and consist of an annular 

 process, which projects transversely from the ectocyst into the interior of the cell, with a 

 covering of endocyst on its upper and under surface (PI. X, fig. 4, b'). The septum thus 

 formed is rendered complete by the aperture in its centre being closed by a peculiar body, 

 which projects into the cavity of the cell at each side. The structure of this body is remark- 

 able ; it consists of a central mass, surrounded by a distinct layer of somewhat elongated 

 cellules placed perpendicularly to its surface. The body which thus closes up the centre of 

 the annular septum has, without doubt, some oflice to perform besides that of simply com- 

 pleting the septum ; but upon the nature of this oflice, or the exact signification of the body 

 itself, I can form no satisfactory opinion. In the other genera the septa are by no means so 

 constant or complete as in Paludicella. In several species of Plumatella, especially P. coralloides 

 (PI. VII, figs. 2, 3), septa exist, but these generally occur only at intervals, leaving several cells 

 between them, \\ hich communicate freely with one another : not unfrequently the septum 



* Dumortier, Eecberches sur I'Anat. et Physiol, des Polj'pes Comp. d'eau douce. ' Bui. de 

 I'Aciid. Roy. de Bruxelles,' 1835. Fig. reproduced in Dumortier and Beneden, Hist. Nat. des Pol. 

 Comp. d'eau douce. 'Mem. de I'Acad. Roy. de Bruxelles,' 1848. Compl. t. xvi. 



t Van Beneden, Eeclierches sur les Biyozoaires tluviatiles de Belgique. ' Mem. de I'Acad. Roj'. 

 le Bclg.,' 1818. 



