Structure of Mngelona. 449 



longitudinal sections, but a special longitudinal coat has not 

 been made out, either in transverse or longitudinal sections. 

 The longitudinal muscular coat of the dorsal blood-vessels, 

 and perhaps the ventral also, may, with the nature of the 

 vrall itself, aid in this respect. The internal surface from 

 the tenth segment backward is probably covered with cilia 

 (thougli they could not be made out in front), and they are 

 well seen (though not so boldly marked as in Nerine and 

 Spio) near the tail. The canal, retaining the same structure, 

 though the circular coat becomes more distinct, diminishes 

 toward the tip of the tail, and terminates in an anus on the 

 dorsal surface a little within the margin. The latter part of 

 the tract shows very active contractions, and the cilia at the 

 anus are largely developed. 



The food consists of sand containing various minute 

 nutritious particles, translucent chitinous fragments of 

 crustaceans, foramiuifera, and other debris. Sand is very 

 necessary for the existence of this form, for though the 

 animals survive a considerable period in captivity in vessels 

 filled with pure sea-water, they thrive much longer amongst 

 fine sand, with a few inches of water over it. No parasites, 

 other than an unknown ovum, have been seen in the ali- 

 mentary canal — a very different condition from that of its 

 congener Polydora ciliata, which has many Gregarinse in the 

 same organ. 



VI. The Nervous System. 



The central mass of the nervous system lies above the 

 pre-oral chamber — and rather in front of the muscular 

 blood-vessels of the part — in the form of two superior 

 ganglia, and they are by no means easily made out in the 

 living animal. Each consists of amass of nerve-cells — some 

 of considerable size, with a commissure of fibres — best 

 marked posteriorly. In the hypodermic region below and 

 in front of the chamber, another (smaller) nervous mass is 

 seen in longitudinal section, but the actual connection be- 

 tween the superior and inferior divisions has not yet been 

 made out. From the oblique appearance of the nerve-cords 

 in section, they certainly pass very rapidly down to the sides 

 of the body from the central mass, and in the living animal 

 their course outward is similarly abrupt. There is no trace 

 of eye-specks, but the animals are sensitive to light, and 

 touch is highly developed. 



When the nerve-cord reaches the lateral region it is found 

 under the channel leading to the tentacle, and somewhat 



Ann. & May. X. Hist. Ser. 8. Vol. vii. 30 



