CIRCULATORY APPARATUS OF THE NEMERTEA. 79 
more numerous are the paired ducts; the higher the types the 
fewer, till they are reduced to one pair. 
The nephridial system of the Hoplonemertea also shows 
several degrees of complexity. It consists of numerous small 
canals, which are connected with a longitudinal canal on each 
side. These send their excretory ducts outwards. The excre- 
tory ducts are either two in number (D repanophorus, 
Nemertes, Malacobdella, Amphiporus pulcher, &c.) 
or more [Amphiporus lactifloreus (and hastatus?)]. 
Where two occur these may be quite anteriorly situated 
(Amphiporus pulcher), in the middle (Drepanophorus 
rubro-striatus), or posteriorly (Malacobdella). 
Thus the nephridial system of all the Nemertea consists of 
one or more canals, directly communicating or not with 
the vascular system, provided or not with cilia, and com- 
municating with the exterior by means of excretory ducts. 
These excretory ducts all lie above the nerve-trunks. 
I now wish for a moment to summarise the very provisional 
hypothesis, which I have ventured to make respecting the 
physiological function of the two proboscidian sheath-vessels 
in the Paleo-type of the median vessel, lying in the probosci- 
dian sheath in the two other types, and of the membranaceous 
sacs of Drepanophorus. 
McIntosh was the first who, both in the Schizo-type and 
the Palzo-type, mentioned a vessel which lay in the probosci- 
dian sheath. Dewoletzky was the first to mention this median 
vessel in the sheath in a representation of the Hoplo-type 
(Drepanophorus). In Lineus marinus, in the foremost 
part of the cesophageal region, McIntosh found one median 
vessel lying in the sheath, and described in Carinoma 
Armandi two vessels in the sheath. In his description of the 
latter he says: “ The physiological signification of these two 
vessels is clear since the discovery of a regular series of vessels 
which communicate with the proboscidian sheath.” 
This passage can hardly be said to clearly explain what 
physiological significance McIntosh ascribes to this arrange- 
ment, and as far as it applies to the existence of an open com- 
