26 E. Ray Lankester, on the Gregarinida. 
to form a layer beneath the investing tunic^ intermediate in 
density as well as position_, which in one or two cases be- 
comes considerably developed. This occurs in the Mono- 
cystis of the annelid Nereis (figs. 4^ 5), where the granules 
occupy a smaller portion of the sac than is usual^ and the sar- 
codic substance in which they are imbedded becomes very 
remarkably diflPerentiatedj so that there is a broad fleshy pro- 
longation of the sac at one extremity, exceedingly mobile, 
which indicates the direction in which progression is always 
made. Distinct striations, giving the appearance of fibrilla- 
tion, may be detected in the substance of this prolongation. 
It seems that here that portion of the viscid material filling 
the sac which is nearest to the enclosing membrane is denser 
than is usual, and has much of the character of sarcode, 
while the granules, which are excessively fine, float closely 
packed together in the inner portion of the same viscous 
material, which is less dense. This species of Monocystis, it 
should be remarked, is very active. This, again, would tend to 
show that the development of granules is in opposition to the 
activity of the animal, which is further borne out by the fact 
that young Gregarinse, in which there are but very few gra- 
nules, are always by far the most active. The striations on 
the investing membrane, which are noticeable in many species, 
such as M. SerpulcBy M. SabellcB, &c., are similar to those 
occurring on the tunics of many Infusoria. In some species 
they occur in immature specimens only, and are not traceable 
in fully grown individuals. This is the case in M. Terebellce, 
and in an undescribed form abundant in Cirratulus borealis 
(figs. 8, 9), while in certain stages of the development of ilf. 
Lumbrici a series of filamentous processes, or sometimes of 
small conical bodies, appear to be developed from the exterior 
of the investing membrane and afterwards cast oflf."^ The 
prolongation of part of the sac into a proboscis provided with 
hooks or a broad flattened extremity, as in G. Sieboldii and 
G. Heerii, also shows the plasticity of this portion of the saccule 
constituting a Gregarina. The movements of a Gregarina 
do not depend on the mere elasticity of the envelope, but 
on the contractions of the dense portion of the viscid sar- 
codic substance contained by it, which is continuous with it, 
and the development of which is opposed to the development 
of the granules. 
Specific distinctions. — It is a matter of very great difficulty 
to decide on specific diff'erences in higher animals possessing 
many more points of character than can be found among Gre- 
garinida, and, indeed, among these latter it becomes almost 
* These filamentous bodies do not form part of the Gregarina, but are 
sperm-cells of the Lumbricus, in M. Lieberkiihn's opinions. 
