PORELLA. 321 



incrusting, or erect ; foliaceous ivith a single layer of cells, 

 or ramose. 



Throughout this very natural group there is a striking 

 uniformity, not only in the characters of the adult cell, 

 but also in the course of its development; and the study 

 of it has done more than most things to convince me that 

 in this section of the Polyzoa we cannot safely regard 

 the mere erect and branching habit as a generic criterion. 

 If we examine a colony of the common P. concimia, we 

 find in the marginal cells a plain semicircular opening, 

 without any trace of the peculiar structure which is cha- 

 racteristic of the adult. In those which are somewhat 

 older, the eel I- wall begins to rise round the primitive 

 aperture, and the outline of a secondary orifice is trace- 

 able, which extends for some distance below the former, 

 narrowing off downwards, and so taking on the very 

 distinctive shape which belongs to this species. As deve- 

 lopment proceeds the wall of this secondary orifice rises 

 higher and higher, until it completely closes in the original 

 opening and forms below it a chamber in which the 

 avicularium is ultimately lodged. The primary semicir- 

 cular aperture is covered by the operculum, and continues 

 to be the real entrance to the interior of the cell ; the 

 secondary formation is an outwork, as it were, which to 

 some extent protects the entrance, and affords a site for 

 the avicularia. 



We have precisely the same course of development in 

 all the species which are grouped under this genus. 



This arrangement coidd not stand, as these forms are most nearly allied, 

 and cannot be separated in a natural system. In his later paper, on Poly- 

 zoa from Nova Zembla, he transfers P. l(Bvis to EscJiara, associating it with 

 P. compressa and one or two other species. Porella, however, which is 

 not encumbered with inconvenient associations, and which was first adopted 

 by Smitt himself for part of the group, seems to me to be in all respects 

 a preferable name. 



