According to the foregoing account, the evidence of carefully executed injections strongly favours the view that a continuity exists between the contractile vascular system and the noncontractile sinus system in Hirudo. This continuity is proved to exist in various regions of the body by means of serial sections. The communication takes place through the capillary systems.
The hæmolymph system of Hirudo consists of four main longitudinal trunks, sending out transverse branches to the body-wall. The dorsal branches of the lateral vessels pass into small annular vessels communicating with the plexus of minute capillaries in the epidermis. From these, again, arise capillaries going to small sinuses which run into the lateral transverse sinuses, and so into the dorsal sinus.
Similarly (he ventral sinus sends annular sinuses along the ventral region of the body-wall opening into the epidermal plexus, whence arise capillaries joining the latero-abdominal vessels.
Continuity between the two systems has also been shown to take place by means of capillaries on the wall of the alimentary canal, and probably exists on the other internal organs of the body.
Two questions still remain to be solved: firstly, as to the circulation of the hæmolymph; secondly, as to the exact homology of the channels in which it flows.
With respect to the first of these problems, I have no direct observations to record; but it may be pointed out that the presence of the valves described above show, at least, that the hæmolymph must flow in a constant direction--that there is a real circulation, not a mere motion backwards and forwards. It seems to me extremely probable that the annular vessels collect the oxygenated blood from the epidermal plexus, and carry it into the latero-dorsal and latero-lateral vessels, whence it would be pumped into the lateral vessels. From these some of the hæmolymph must be carried by the latero-abdominal vessels to the various organs of the body, and to the ventral cutaneous plexus. The annular sinuses would collect it from this plexus and carry it into the ventral sinus. The abdominodorsals and the dorsal sinus would appear to supply the dorsal and lateral cutaneous plexus.
We are left in considerable uncertainty as to the true nature of some of the spaces. That the lateral vessels belong to the real vascular system, and that the ventral sinus and perinephrostomial sinuses belong to the true cœlomic system, seems to be clearly established both by comparative anatomy and by the embryological researches of Bürger (2). This observer, however, could not trace the dorsal sinus to a cœlomic origin, and since its branches bear the same relation to the cutaneous plexus as those of the latero-abdominal vessels, I am inclined to think that the dorsal sinus may represent the dorsal vessel of other annelids. In that case the cœlomic cavities do not persist dorsally, or have never reached the median dorsal region in the Gnathobdellidæ.
The annular channels may possibly represent the annular cœlomic lacunæ so well described and figured by Oka in Clepsine (10), and it may perhaps be through them that the chief communication between the cœlom and the vascular system has been established. The observation of the some-what variable relations of these annular channels tends to support this view.
With the very imperfect knowledge of the development of the cœlom and blood-vessels in Hirudo at our disposal, we cannot say for certain at present where the one ends and the other begins, nor whether a given capillary really belongs to the one or the other. Nor can we safely conjecture how the continuity has actually taken place. But one thing seems fairly certain, namely, that it is not only by means of the botryoidal channels that the communication has been brought about.
It is very tempting to compare the leech with the Vertebrate, in which a third system of spaces--the lymphatic system--has been interpolated, allowing a communication to take place between the originally distinct cœlom and blood-vascular system.1 But the botryoidal tissue is not so inter-polated in the case of Hirudo; if it were obliterated, the two systems would still be in free continuity by means of capillaries. The botryoidal channels would seem to be rather of the nature of a by-path, through which the hæmolymph does not necessarily circulate. In this connection it should be mentioned that in sections they are rarely seen to be as much distended with the fluid as the neighbouring capillaries of similar size.
Whatever may be the process whereby the continuity between the cœlom and vascular system has been established in the Gnathobdellidæ, there can be little doubt that it is a secondary condition, and that the structure of such a form as Acanthobdella, in which a closed blood-system lies in a normally developed cœlom, is really the more primitive.
Modeling the Body of the Embryo during the Somitic Period