120 ZOOLOGY 
It is stated to flow forwards in the lateral, and backwards in 
the dorsal vessel. 
The nephridia of Nemertines are paired and situated 
anteriorly (Fig. 77). Their ducts open to the exterior by one 
or more openings on each side of the body, and they always 
lie above the nerve trunk. These ducts are lined by a single 
layer of ciliated epithelial cells, and are sometimes much 
branched ; their inner ends vary a good deal in different genera, 
and there is considerable discrepancy in the accounts of 
different observers. In the freshwater TZetrastemma the ends 
of the branched ducts are said to terminate in flame cells, and 
Fic. 80.— Diagrammatic sections to 
show disposition of internal organs 
in (I.) Carinella, a Palaeonemertine, 
and (II.) a Schizonemertine. 
1. Cellular portion of integument. 
2. Basement membrane. 
3. Circular muscular layer. 
4, Longitudinal muscular layer. 
5. Second circular muscular layer in I. 
6 
. Second longitudinal muscular layer 
in IT. 
7. Nervous layer. 
8. Lateral nerves. 
9. Cavity of proboscis sheath. 
10, Proboscis or introvert. 
11. Intestine. 
12. Lateral blood-vessels. 
13. Dorsal blood-vessel. 
14. Connective tissue. 
the whole system is compared to that of Turbellarians. The 
system is embedded in the parenchyma. In Carinella a portion 
of the wall of the lateral blood-vessel is modified to form the 
internal end of the nephridium. This forms a spongy gland 
which is continuous with the duct, the latter is also stated to 
open at two points into the blood-vessel. 
The central nervous system consists of two pairs of ganglia 
in the head united by two commissures. One of these, the 
