424 ALFEED GIBBS BOURNE. 



species which may be referred to the genus Pontobdella at 

 Naples, but only one of these is at all common, the others are, 

 indeed, exceedingly rare, four or five specimens only having 

 come to hand during several years ; this small number is, how- 

 ever, probably owing to the diflBculties which exist in searching 

 for them in the fish market. I propose now to speak of one 

 species only, Pontobdella muricata. 



The ordinary somite here presents the following characters : 



1. External. — There are four annuli marked by rings of 

 papillae ; other annulations may be present, but careful exami- 

 nation and comparison of numerous specimens show that these 

 are secondary in nature, due to a state of contraction. 



The first annulus (fig. 6.) (i) presents eight large papillae, 

 i. e. papillae which, when extruded, are large, or when flattened 

 present a large basal marking. These are arranged at regular 

 intervals, except that in the median dorsal and ventral lines 

 there is a more marked gap, so that they may be considered 

 to be arranged in two lateral groups of four on each side. 



The second annulus (ii) presents twelve papillae, smaller than 

 those in the first annulus ; of these, four lie in a group in the 

 median dorsal region, and the remaining eight form a con- 

 tinuous series around the lateral and ventral regions of the 

 annulus. Between the dorsal group and the others lies on 

 each side a smaller papilla slightly posterior in position to the 

 series. 



The third annulus (iii) is much narrower, and its papillae 

 much smaller ; they are twelve in number and thus grouped : — 

 Two near together in the median dorsal line, a gap, and then 

 one on each side, another gap, and then two more, the remaining 

 two being median and ventral in position. 



The fourth annulus (iv) resembles the second in all points, 

 except that the lateral ventral series contains only six papillae 

 instead of eight, there being altogether ten large and two small 

 instead of twelve large and two small. 



Such is the normal somite, and this arrangement obtains 

 with exceeding regularity ; here and there a little shifting in posi- 

 tion may occur or small extra papillae be visible, but very rarely. 



