AXD OYIDTJCAL SYSTEM IN THE LAMELLIBEANCHIATE MOLLHSKS. 
39 
or other injection has been thrown in by the orifice through which the generative pro- 
ducts are extruded, a simple lens is sufficient to show that the tubes thus injected 
have the generative caeca afiixed to them laterally, and pass on continuously into parts 
of the foot in which no generative caeca are lodged. It is most especially in that part 
of the muscular foot into which no viscera are packed, and which forms a belt of con- 
siderable width beyond and bounding the generative mass, and yet free from any admix- 
ture of its constituent elements (as the microscope will show), that we find the capillary 
network (shown to be in connexion with the oviducal outlet whilst clear of the terminal 
caeca) of the gland to attain its maximum development. Now this area is the area also 
of maximum distention in the distended foot. If in an TJnio which has been injected 
from the blood-vascular system and from the oviducal, both with difierently coloured 
injecting-fluids, a portion of the injected tissue be taken from this area and placed 
under one of the higher powers of the microscope, the fluid which has been thrown in 
by the oviducal orifice will be seen to be contained in tubes as well and sharply defined 
as those of the capillaries which the other injected fluid will show to be in connexion 
with the blood-vessels. 
"Wliilst the analogy of the Echinodermata and many Annelids does away with any 
a priori improbability which may have seemed to attach to the possession by these 
mollusks of the system of tubes the existence of which we have been striving to demon- 
strate, the homology of the Brachiopoda furnishes us with a strong a priori presumption 
in favour of the correctness of our view. On the other hand, we cannot forbear point- 
ing out the great improbability which must attach to a view which supposes a fluid of 
such morphological and such chemical characters as is the blood of the freshwater 
mussel to be diluted as it must be diluted on the hypothesis of the blood-vessels being 
the agents by which the animal voluntarily distends itself often to thrice its undistended 
bulk. How do the blood-corpuscles which we may take from the interior of the animal’s 
heart behave when thus mixed with water under the microscope * '? But it is not upon 
considerations such as these that we would lay most weight, but upon the evidence 
which injections of the several systems fimiish to the unassisted eye, and upon the con- 
firmation of that e\idence which microscopic inspection furnishes. 
* “ Eeagentien, wie ein UelerscTiuss von Wasser, Terdiiimte Essigsaure, losen bei der ersteren Art (Blut- 
kdrpercben) den scheinbar festen Inbalt auf, und lassen den Kern, wie die eingescblossenen Korncben, 
deutlicb bervortreten. Ibre baufigen Eormveranderungen, z. B.-die “ spiessigen birscbgeweibabnbcben 
Eortsatze,” welcbe sie treiben, bangen von unvermeidlichen Diffusionverhdltnissen ab, welcben sie bei der 
grossen Wassennenge gegeniiber ibrer verbaltniasmassig geringen Anzabl ausgesetzt sind. Wabrend A. 
Eckee dieselben durch erne Bilcking von Vacuolen, in Folge deren Yergrdsserwng sie einreissen, zu erklaren 
Bucbt, bait Lieberkiibn diese Zellenbildungen fiir Amoben mit selbstandigen contractilen Bewegungen. In 
innigem Zuaammenbang mit dieaen Eracbeinungen atebt das leichte Austreten des Zelleninhalts, welcber 
biaweilen in bellen und byalinen Tropfen berumacbwimmt, ja nicht selten geJit ein Zerfallen desselben in 
zablreicbe kleine Tropfcben nocb innerbalb der Zellen vor aicb, welcbe dadurcb ein maulbeerartiges Anaeben 
bekommen, ebenao vereinigen die auagetretenen Sarcodetropfen dieae Kbrpercben zu den oben erwabnten 
Kliimpchen und Elockcben.” — Vox Hesslixg, loc, cit. pp. 219, 220. 
