E. RAY LANKESTER, ON GREGARINID&. 91 
of the proboscis (fig. 2 a). Within this the second membrane 
can be plainly observed, enclosimg the granular mass, and 
extending within the proboscidiform appendage, but not in- 
voluted as is the external membrane (fig. 2 6). In some va- 
ricties this second tunic is still more evident, being contracted 
within the external membrane, and exhibiting striations 
(fig. 3). In the list appended to this paper, I have named 
this species after the annelid which it mhabits. 
In various species of Sabella, I have met with many uni- 
locular Gregarinid of an elongated form, measuring from 
—7th to +2,,th of an inch in length (fig. 15,16). This species, 
which I propose to call Monocystis Sabelle, differs considerably 
from that found in Serpule, being much longer in proportion 
to its breadth, and attenuated at one extremity. There are 
but few granules in the sac, and very indistinct striations on 
the surface ; a well-defined vesicle, generally without any con- 
tents, is always present. The species of Sabella I examined 
were S. alveolata, (Amphitrite) bombyx, inpendiculum ; but the 
Gregarine appear to belong to one species, and present no 
difference in structure or form. 
On account of the great mutability of form which is cha- 
racteristic of the Gregarinz, the attempt to divide them into 
families, genera, and even species, is attended with consider- 
able difficulty. In 1838,* Dr. Hammerschmidt placed certain 
new forms of Gregarina in four genera, Clepsidrina, Rhizinia, 
Pyxinia, and Bullulina, scarcely assigning his reasons for so 
doing. Kélliker did not make any division of the species he 
described, but left them all in Dufour’s genus Gregarina. In 
1845, Dr. F. Stein, believing Hammerschmidt’s genera to be 
entirely unsatisfactory, proposed the following classification 
of the species then known, dividing them into three families 
and seven genera, thus : 
Family. Monocystidez ; unilocular Gregarine. 
Genus. Monocystis; animals living singly. 
Genus. Zygocystis ; animals living in pairs. 
Family. Gregariariz ; body divided into two portions by 
a septum. 
Genus. Sporadina; single animals, without an ap- 
pendage to the head. 
Genus. Stylorhynchus; single animals, with a probo- 
scidiform appendage to the head. 
Genus. Actinocephalus; single animals, with an ap- 
pendage to the head, furnished with hooks. 
* ‘Tsis von Oken,’ 1838, p. 356. 
