50 



POLYPIFERA. 



ramifications may be extended. As the growth 

 of the tegumentary layer thus proceeds in one 

 direction only, except when the development 

 of polype-cells calls for its lateral expansion, 

 the nutritive layer within continues to grow 

 pari passu, and from it the polypes are pro- 

 duced as the cells become ready to receive 

 them. 



BRYOZOA (Ehrenberg), CILIOBRACHIATE 

 POLYPI (Farre). The Bryozoa, although 

 closely resembling some of the simpler Poly- 

 pifera, described in the preceding pages, with 

 which, indeed, until a very recent period, they 

 were confounded by zoological writers, differ 

 from them in so many essential points of their 

 structure, that, but for the convenience of 

 description, we should have preferred to re- 

 gard them as a distinct class, exhibiting a 

 much higher phase of organisation than any 

 of the nudibrachiate races. In all the fami- 

 lies of Polypifera we have as yet had occasion 

 to examine, it will have been noticed that the 

 tentacular apparatus around the mouth, al- 

 though very generally pinnated, are quite 

 devoid of cilia ; but in the Bryozoa one 

 of the most obvious circumstances observ- 

 able in their organisation is, that all the 

 circumoral arms are crowded with vibratile 

 organs, the play of which, when in action, is 

 exceedingly energetic, producing rapid cur- 

 rents in the surrounding water, and thus 

 hurrying towards the mouth of the animal 

 whatever substances may come into the neigh- 

 bourhood of the vortex so produced, and in 

 this way securing an abundant supply of food, 

 almost without exertion on the part of the 

 creature itself. From this most conspicuous 

 character, common to the entire group, Dr. 

 Arthur Farre was induced to propose for 

 them the name of Ciliobrachiata. It is in 

 their internal economy, however, that their 

 chief points of distinction are to be sought. 

 Like the ordinary polypes, most of these little 

 animals inhabit cells of different shapes and 

 various degrees of density. These cells are 

 sometimes calcareous and opaque, but in very 

 many genera so thin and diaphanous that 

 nothing is more easy than to examine, by 

 means of the microscope, the anatomy of the 

 animal within. When thus examined, the 

 differences between a Bryozoon and an ordi- 

 dinary polype become immediately manifest, 

 and may be briefly stated as follows. 



In the nudibrachiate polypes the stomach 

 is a simple sacculus unprovided with any 

 intestinal tube or anal orifice, so that after 

 taking food the egesta are necessarily ex- 

 pelled through the oral opening ; but in the 

 Ciliobrachiata, not only is the stomach found 

 to be floating loosely in a visceral cavity, and 

 of very complete structure when compared 

 with the digestive sacculus common to the pre- 

 ceding tribes, but it terminates in a complete 

 intestinal canal, provided with a distinct anal 

 orifice, through which the fasces are discharged. 

 Accompanying this advanced condition of the 

 alimentary apparatus all the other systems 

 assume a more elevated type of structure, as 

 will be immediately apparent from the details 



of their anatomy, upon the consideration of 

 which we are about to enter. Much, doubt- 

 less, yet remains to be made out in the eco- 

 nomy of these animals ; still the researches 

 of Ehrenberg*, Milne Edwards, Audouinf, 

 Thompsonj, Farre , and Van Beneden ||, have 

 already put us in possession of most important 

 information concerning them, which promises 

 to open a yet wider field for discovery. 



The cell of Bowerbankia (fig. 56), as de- 



Fig. 56. 



Bowerbankia densa, magnified 80 diameters. 



a, one of the animals fully expanded ; 1, pharynx ; 

 2, cardia ; 3, manducatory organ, or gizzard ; 4, sto- 

 mach, its parietes studded with the hepatic follicles ; 

 5, pylorus ; 6, intestine, containing pellets of feculent 

 matter ; 7, anus. The gastric (8) and tentacular (9) 

 retractors are seen within the cavity of the body. 

 The flexible portion of the cell, or the operculum, 

 is seen expanded and surrounding the upper part 

 of the body. 



b, a similar animal completely retracted. The 

 stomach drawn to the bottom of the cell. The 

 upper portion of the alimentary canal flexed. The 

 tentacula somewhat distorted by the pressure of the 

 operculum. Their retractor filaments (1) relaxed. 

 The upper part of the cell is occupied by the oper- 

 culum folded up in its axis, and from it the upper 

 (2) and lower (3) sets of opercular retractors are 



* Symbolae Physicae. 



t Annales des Sciences Naturelles, for Sept. 1828, 

 and July 1836. 



t Zoological Researches, Mem. V., Cork, 1830. 



Phil. Trans, for 1837, part 2. 

 J| Recherches sur 1' Anatomic, la Physiologie et 

 I'JEmbryogenie des Bryozaires. 4to. Brussels, 1845. 



