AND ITS BIOLOGICAL SIGNIFICANCE. 



329 



chief parts, a long distal rod, the rnanubrium (Fig. H, 2, mab.), and a shorter proxi- 

 mal portion, the uncus (un.). The incus or central portion consists of three main 

 parts. There is a long median curved rod, the fulcrum (Jul.), which bears at its prox- 

 imal end two large structures known as the rami (rm.). These articulate with the 

 fulcrum and enclose a space between them. At their proximal ends they bear a num- 

 ber of teeth. The rami have their lower or distal end produced into a long process, 

 which with Gosse we may call the alula (nl.). 



In the trophi of Rattulus longiseta Schrank (Fig. H, 2) it will be noticed that 

 the left side is somewhat better developed than the right. The left manubrium is 

 stouter, the left uncus is thicker and bears teeth, while the right one is slender and 



8 



FIG. H. Trophi of a number of species of Rattulidse, dorsal view (except 1) (original). Abbreviations: al., alula; 



ful, fulcrum; mob., manubrium; rm., ramus; un., uncus. 



1, Rattulus longiseta Schrank, side view; 2, the same, dorsal view; 3, Rattulus bicuspes Pell; 4, Rattulus 

 bicristatus Gosse; 5, Rattulus elongatus Gosse; 6, Diurella sulcata Jennings; 7, Uiurdla tcnuior Gosse; 8, Diurella 

 poreellus Gosse; 9, Rattulus mucosus Stokes; 10, Diurella tigris Miiller. 



without teeth; the left alula is longer than the right. These differences are char- 

 acteristic for the whole family, but are much more pronounced in some species, as a 

 glance at Figure H will show. In such cases as Diurella sulcata Jennings (Fig. H, 6) 

 and D. tenuior Gosse (Fig. H, 7) the right side is much smaller than the left, while in 

 D. tigris Miiller (Fig. H, 10) the right manubrium and uncus have almost disappeared. 

 Altogether, then, we find that all the Rattulidse are more or less unsymmetrical 

 in many features of their body structure. If we seek for a general statement which 

 shall express the nature of this asymmetry, we shall find it most fully set forth as fol- 

 lows: Conceiving the middle of the body to be a fixed point, the anterior part of the 

 body seems to be twisted to the right, the posterior part twisted to the left. 



