BY E. J. GODDARD. 723 



be readily made out in this region. The last five annuli of the 

 neck are constituted by the last annulus of somite iv. of that 

 region, as made out by noting the annuli visible, three annuli of 

 somite v., and the first annulus of somite vi., which is the most 

 anterior somite of the trunk-region. The middle annulus of 

 somite v. differs in no way, as regards the tubercles, from that 

 anterior or posterior to it. 



Somites. — As has been already stated, the somite is triannulate; 

 and the somites can be readily made out, owing to the fact that 

 between any two adjacent small annuli runs a strong transverse 

 sulcus, and this occurs regularly throughout the body. One can 

 safely conclude that this marks off the limits of the somites, and, 

 in accordance with this, the large annuli, provided with their 

 prominent tubercles, constitute the middle and second annulus 

 of a somite. These tubercles correspond to the more typical 

 papillae in other species of Pontohdella^ and have the same 

 metameric significance. This, then, is in keeping with Castle's 

 generalisation, that the annulus bearing sensory papillae consti- 

 tutes the middle annulus of a somite. There can be no doubt 

 that, in this species, such is proved to be the case; but, as will 

 be shown later, this does not hold in all members of the Hiru- 

 dinea, the sensory annulus really denoting the most anterior of 

 each somite, as suggested by Whitman, in some forms. 



The arrangement of the annuli, with regard to somites, as seen 

 in P. macrothela, is as follows : — 



Somite. Nature. 



Neck 



Triannulate. 

 Triannulate. 



Trunk. { 19-51 vii.-xvii. Triannulate. 



xviii. Biannulate. 



xix.(?) BiannulateC?). 



From this it will be seen that there is no abbreviation of the 

 somites noticeable at the anterior extremity; and that abbrevia- 

 tion is noticed in somite xviii., where the first annulus is that 



