OBSERVATIONS ON POLYZOA. 



223 



doubt of their being what I have stated. The larger cells are 

 continually multiplying by division, and there result num- 

 bers of small cells which lie scattered here and there in the 

 supposed blood channels. Fig. 3 shows a large cell under- 

 going the\process of division, and below, near the right 

 lower corner of the figure, there are two minute cells, un- 

 doubtedly created in a similar manner, occupying the in- 

 terstices of the membrane. Fig. 5 shows a group of cells 

 taken from a point nearer the orifice than those of fig. 2, 

 and, also, from a different zooid. These are not so dis- 

 figured by contraction and have more angular outlines. 

 Fig. 4 shows a group of five cells, from another zooid, more 

 highly magnified than either of the above, and more wide- 

 ly separated. When the cells are so dispersed the intervals 

 are usually more or less filled in by minute cells ; but, in 

 this instance, the spaces were vacant and the nucleus of 

 immense size, the nucleolus not being visible. 



FIG. 2. FIG. n. FIG. 4. FIG. 5. 



FIGS. 2 and 5, groups of cells of the first membrane, greatly enlarged, from the 

 cceiHccium of I'LUMATKLLA VITREA. 



FIG. 3, one cell still more enlarged showing nucleus and nucleolus. 



FIG. 4, E', cells of first membrane: E'", muscular fibres of the third layer: E'">, 

 muscular fibres of the fourth layer. 



The cells on the coenoecia of Pectinatella and Cristatella 

 do not differ sensibly in their structure from those of Fred- 

 ericella and* Plumatella. The outer sides, however, being 

 free from the presure of an ectocyst, are more convex ; and 

 the longitudinal diameters, instead of being less, are greater 

 than in the cells of the same membrane in the evaginable 

 endocyst. Plate 13, fig. 16, and figures 7, 8, E' present lat- 

 eral views of the membrane in the coanrecia of Cristatella 

 and Plumatella: in figure 8 the cellular structure is not 

 given, but the relative thickness of the membranes may be 

 estimated by a comparison of the two figures. The cells of 

 the first membrane of the evaginable endocyst do not vary 



