11 



Anatomy. 



A. Orijans for the Preservation of the Individual. 



( 1 ) Dermal System. 



Coenwcium. — The Polyzoa being all composite animals, the coenoecium constitutes essen- 

 tially an assemblage of little cells or chambers, of very various form, organically connected 

 with one another. Each chamber lodges a polypide, and its cavity is either shut off from 

 those of the neighbouring chambers, or freely communicates with them. In every instance* 

 the polypide can be protruded from its cell, and again withdrawn into it, and the part through 

 which it thus passes outwards and inwards, as has already been defined, is the orifce of the 

 cell. It must not, however, be supposed that there is here any proper orifice, the retraction 

 and exsertion consisting merely in an invagination and evagination of the anterior part of 

 the cell. 



The coenoecium, in every case, except in Cristatella, is composed of two distinct mem- 

 branes, which must be carefully distinguished from one another — an internal, the endocyst, 

 which is always soft, transparent, and contractile ; and an external, the ectocyst, which varies 

 greatly in character in the different genera. 



The endocyst (PI. Ill, fig. 7 ; V, figs. 5, 6 ; IX, fig. 7 ; X, fig. 4, a) lines the interior of 

 the cells, and when it arrives at their apertures would protrude beyond the ectocyst were it 

 not that it here becomes invaginated or inverted into itself, and then terminates by being- 

 attached round the base of the tentacular crown. During the exsertion of the polypide, the 

 invaginated portion of the endocyst is carried out with the latter, thus undergoing a process 

 of evertion, which, however, in all the fresh-water species, is but partial, a portion of the 

 endocyst, as we shall afterwards more particularly see, remaining in a permanently inverted 

 condition ; in this respect differing remarkably from the marine species, in which the evertion 

 of the endocyst is, perhaps, in all cases, if we except the anomalous genus Pedicellina, 

 complete. The attachment of the endocyst to the base of the tentacular crown closes the 

 coenoecial chambers externally, while the polypide is thus suspended in the midst of the 

 fluid with wliich these chambers are filled. , 



If we examine the endocyst histologically., we shall find that it possesses a very distinct 

 structure. In Lophopus, which is particularly well adapted for observing the intimate structure 

 of this membrane, we find it composed of large irregularly shaped cells, widely separated from 

 each other by an intervening substance towards the posterior part of the coenoecium, but more 

 closely approximated towards the orifices. These cells are filled with a perfectly colourless 

 and transparent fluid. Under the action of acetic acid each is distinctly seen to be bounded 

 by a doul)le outline, and to contain a large nucleus with nucleolus (PI. II, fig. 9) ; the 

 nucleus, with its nucleolus, are imbedded in the walls of the cell. The intervening substance, 

 which, before the application of the acetic acid, appeared simply granular, is now seen to 



* In Pedicellina, and apparently also in Urnatella, the power of protrusion and retraction is 

 very imperfect, and is here limited to such change of position as is connected with a slight extension 

 and flexion of the oesophagus. 



