810 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



worms liad been in captivity three months, and although 

 apparently fairly healthy, were less vigorous than at my 

 first experiments. Neither worm showed the excitement 

 or activity of the first occasion. The first specimen refused 

 to replace a second time the anterior valves and began to 

 fail in health ; but the other worm replaced the posterior 

 pair (alternate valves) one at a time, three or four 

 days, apparently, being occupied in the formation 

 of each. 



Returning to the structure of the tube itself, two 

 features in the arrangement of the valves therein are at 

 first sight surprising, (1) the perfect alignment of all the 

 valves (both front and back), and (2) the difference in 

 arrangement of the former and latter, and the mathe- 

 matical accuracy with which the valves are shaped. An 

 explanation of all these wonders may, perhaps, be found 

 in the form of the tube itself, which not only tapers con- 

 siderably from one end to the other, but is also more or 

 less elliptical in cross-section. The free edges of the 

 valve-membrane always represents the major axis of the 

 ellipse, and it seems not improbable that the animal may 

 itself usually assume a definite position inside the tube in 

 relation to this axis, and thereby be guided in oriental ing 

 the valves as described. The valves at both ends of the 

 tube, it will be remembered, are constructed upon the 

 same principle, namely, that of the valve in a vein, but 

 the pockets at the smaller or posterior end, instead of 

 being opposite are alternate. It seems to me that this 

 peculiar arrangement, which is somewhat similar to that 

 met with in the smaller veins, may be satisfactorily 

 explained by lack of room in that part of the tube for the 

 necessary free movement of the animal's head. 



Although, so far as I am aware, this interesting defen- 

 sive mechanism tins not been observed in other groups, 



