Miscellaneous. 239 
the history of which possesses so much interest in various ways, 
presents the most remarkable organic arrangements from the point 
of view which now occupies us. 
It bears on its back, placed symmetrically on each side, from 
fourteen to twenty pairs of branchize, elegantly twisted into a spiral, 
between which may be observed an oval pit of a more delicate texture 
and more transparent than the rest of the integument of the body, 
upon the middle of which there rises a small mamilla pierced by an 
orifice like a button-hole. 
If the extremity of the pipe of a syringe be applied to this orifice, 
and an injection be made with much care, so as to avoid injury and 
consequently all causes of error, the coloured liquids or air employed 
are soon seen filling the venous system. If the animals are in favour- 
able conditions, the veins may even be injected merely by directing 
the stream of coloured liquid from a distance upon the mamilla of 
the oval interbranchial fossa. 
The Thetys bearing, as we have just seen, from fourteen to twenty 
pairs of branchize on each side of the body, may therefore at pleasure 
introduce water into its blood, or get rid of a portion of this nutritive 
fluid, in front, behind, or towards the middle of its length, by means of 
from twenty-eight to forty orifices. Hence, when we take up this 
animal well expanded and developed, can we be astonished to see it, 
in the hands which it inundates with fluid, change its form, retract 
itself, and gradually lose more than one-third of its volume? It is 
sufficient, however, to consider this in order to understand that, if the 
animal could not reject a portion of the liquids which impregnate its 
tissues like the water imbibed by a sponge, it would be impossible 
for it to diminish its volume. 
In Thetys leporina, as in all the naked Mollusca the observation 
of which is easy, effects may be produced by irritation which it is 
very useful to study. When a part of their body is touched, it is seen, 
under the influence of irritation, to contract and return upon itself, 
driving off the liquid lodged in the meshes of its tissues, and causing 
the dilatation or inflation of some other part. If the latter be irrita- 
ted, the same thing takes place, it contracts in its turn; and by mul- 
tiplying the points of contact, the blood, being driven in all directions, 
and no longer finding any place in the economy, is forced to escape 
outwards: if orifices exist, it is through them that it issues; and if 
these orifices are insufficient, it ruptures the tissues in order to make 
a passage for itself. Direct observation can leave no doubt upon this 
latter fact. 
But when the issue of the blood is not violent, but natural, it is 
subjected to a veritable appreciation on the part of the animal. Of 
this the organization of the orifices furnishes a proof. In Den- 
talium and Pleurobranchus I have described two muscles and a 
valve which oppose the escape of the blood when the animal does 
not consider it desirable. Here a muscle with circular fibres forms 
a sphincter sufficiently developed to produce the mamilla of the oval 
fossa. It is indeed this sphincter, which is usually much contracted, 
that retains the liquids injected into the nervous system, and renders 
