I20 



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 Tetrastemma the ends 

 of the branched ducts are said to terminate in flame cells, and 



8- 



14--, 



Fig. 80. — Diagrammatic sections to 

 show disposition of internal organs 

 in (I.) Carindla, a Palaeonemertiue, 

 and (II. ) a Schizonemertine. 



1. Cellular portion of integument. 



2. Basement membrane. 



3. Circular muscular layer. 

 i. Longitudinal muscular layer. 



5. Second circular muscular layer in I. 



6. Second longitudinal muscular layer 

 in II. 



Nervous layer. 



Lateral nerves. 



Cavity of proboscis sheath. 



Proboscis or introvert. 



Intestine. 



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 



