CIRCULATORY APPARATUS OF THE NEMERTEA. 71 
In the posterior sucker of Malacobdella numerous vascular 
branches are developed. 
In all Nemertea the foremost communication of the 
vascular spaces occurs above the proboscidian sheath, the 
hindmost above the intestine. In all Nemertea the blood- 
spaces are everywhere coated internally with a layer of epi- 
thelium, then a layer of hyaline basal tissue follows always. 
Outside this basal layer there may lie either large vesicular 
cells (figs. 70, 71), or protoplasmatic cells with large nuclei 
(figs. 62—65), or circular muscular fibres are formed by these 
cells (figs. 66), or, but more rarely, longitudinal fibres are 
present in addition (figs. 74, 75). 
With regard to the nephridia in Carinella, a portion of 
the lateral vessels is changed into such an apparatus. This 
apparatus here is distinctly a portion of the blood-space, sepa- 
rated from it, just as in the cesophageal region other portions 
detach themselves; it communicates both directly and in- 
directly with the vascular system. Indirectly, because a por- 
tion of the walls of the blood-vessel changes into a gland, the 
function of which appears to be to convey the superfluous matter 
from the blood-fluid towards a reservoir, the portion separated 
from the blood-vessel. This has two open communications 
with the blood-vessel. Thus, the separation is, as it were, 
still imperfect, and the points of communication appear to 
diverge from one another with the growth of the animal. The 
secretions of the nephridial glands must be removed, therefore 
an excretory duct is developed on each reservoir. The two 
excretory ducts lie nearly on the same transverse level, so that, 
if the animal were a segmented one, they would be found in the 
same segment. They are generally contracted by the basal- 
membrane and the skin, so that no sea-water can enter the 
reservoir but by an inner impulse caused by the muscular walls 
of the reservoir the ducts may open. I suppose that the 
removal of the secretions happens periodically. Thus the 
nephridia of Carinella, the lowest type of Nemerteans, in 
which up to this time a nephridium has been found, is highly 
primitive. 
