894 REPORT—1850. 
trace of the spines is visible. These however shortly after show themselves 
growing out from the two faces of the ovum; they penetrate the gelatinous 
envelope, and soon impinge on the external membrane, which by this time 
has lost its cilia, and which now gives way, torn by the grapple-like extremi- 
ties of the spines. The two temporary investments of the ovum now rapidly 
disappear, and the latter presents itself as the elegant little spiny lenticular 
body so characteristic of the genus Cristatella. 
I have observed in the freshwater Polyzoa the curious fact of the very 
same individual producing two different kinds of ova. This occurs in Plu- 
matella emarginata and in Aleyonella Benedeni. In both these, the cells may 
be observed towards the end of summer loaded with ova which lie loose 
within them. These are of an elongated oval figure, with a largely developed 
annulus which overlaps a considerable portion of the disc. But besides these 
bodies, others may also be observed invariably attached to the internal sur- 
face of the walls of the cell, to which they adhere by means of a peculiar 
cement, in which no trace of structure can be detected. These differ also 
from the unattached ova in shape, being much shorter in proportion to their 
width, while the annulus is exceedingly narrow, and presents but slight traces 
of that highly developed cellular structure so remarkable in the others. 
After the decay of the ccencecium, many of these attached ova may be seen 
adherent to the stone or other body on which the specimen had developed 
itself, and to which they are now connected in lines through the medium of 
a portion of the old cell in which they had been produced. I am unable to 
state whether the development in these last-described bodies is similar to 
what occurs in the others, as I have not succeeded in witnessing the escape 
from them of the young. 
In Alcyonella fungosa and Lophopus erystallinus, [have alsowitnessed bodies 
which differed from the ordinary ova of these Polyzoa in the possession of a 
regular elliptical aperture in the centre of their more convex face. They 
were always empty, and of their nature I have not been able to form any 
conclusion of value. 
‘The nearly opake horny investment of the ova of all these species renders 
it impossible to trace those stages of the development of the embryo which 
occur previously to the hatching of the egg; and on rupturing the latter 
under the microscope, nothing can be witnessed but the escape of a fluid 
helding in suspension innumerable corpuscles which spread themselves over 
the field and exhibit but obscure traces of definite aggregation. When 
however the development has goue on to a certain extent within the ovum, 
the latter opens by the separation from one another of the two faces, and the 
young polyzoon gradually emerges and floats away freely through the water, 
but the surface of its investing tunic is altogether destitute of cilia or other 
active organs of locomotion; and its motions through the surrounding fluid 
seem to be quite passive, except so far as they may be possibly influenced by 
the ciliary action of the tentacula. It now possesses all the essential organs 
of the adult, the retractor muscles are well-developed, and the polypide is 
capable of regular exsertion and retraction ; but the ectocyst is colourless and 
transparent, and free from the earthy particles which in the greater number 
of species are afterwards found in it, and the little animal is still simple. It 
loses however no time in developing gemme, which soon change it to the 
perfect compound form of the adult. In many cases the two separated faces 
of the original ovum continue for some time to adhere to the lower end of 
the little animal like the valves of a bivalve shell. 
I have sought in vain, in all the freshwater Polyzoa, for some orifice through 
which the ova may escape from the cells; and yet, from the large size and 
