426 PART II. SYSTEMATIC ACCOUNT 



slight split along the narrow backward-pointing end (Fig. 1635). In the 

 hind part of the metameric section the black horseshoes give way to thin, 

 transverse plaques. As noted by Ushakov (1933) the change-over from one 

 type of plaque to the other may be abrupt in one individual, while in another 

 it may be gradual, so that one may observe an alternation of horseshoes and 

 simple transverse plaques. In one individual, for example, with 22 pairs of 

 horseshoes, on one side of the body there were 20 -horseshoes in sequence, 

 then in succession came a plaque, a horseshoe, two plaques, another horseshoe 

 and then simple plaques; on the other side there were 16 horseshoes, then a 

 regular alternation of plaque with horseshoe until the full 22 horseshoes 

 had been reached. The simple plaques are transverse oval membranes with a 

 heavily thickened front edge which takes the form of a bent bow whose ends 

 are, as it were, lopped off. They are about 60-65/x across. The dorsal ciliated 

 band is well developed (Fig. 163C) and the non-metameric division of the 

 trunk is very long and characterized by scattered papillae which are furnished 

 with simple plaques but no pulvilli (Fig. 2). There is no zone of thickened 

 papillae. The girdles are well developed, but unusual, since there is no dorsal 

 gap and they are interrupted only on the ventral side (Fig. 1645, C).* There 

 are five to seven rows of elongated, oval toothed platelets in each girdle (Fig. 

 165 A). The platelets are much narrower at the front end, which thus assumes 

 the appearance of a small, colourless head-piece, bearing oblique rows of very 

 small teeth all pointing backwards (i.e. the anterior group of teeth). The 

 greater part of the platelet is brownish and its surface is covered with oblique 

 rows of large teeth pointing forwards (Fig. 1655). The platelets are 13-23/x 

 long. The long and slender postannular region of the trunk is cut off from the 

 girdles by a narrow waist (Fig. 1645, C) and bears on its ventral side numerous 

 transverse metameric rows of papillae. In one complete specimen there were 

 59 such rows (Fig. 2) arranged at more or less regular intervals, the length of 

 which, to all appearances, varies with the state of contraction of the dermal 

 musculature. On the papillae lie transverse, rod-like plaques up to 100/x long. 

 The average dimensions of the body, to judge by measurements made on a 

 number of more or less complete specimens, are as follows : length of the 

 tentacular cylinder, 26 mm; length of the fore-part of the body, 2-5 mm; 

 breadth of the mesosoma, 1-2 mm; length of the metameric part of the pre- 

 annular region, с 30 mm; length of the non-metameric part, с 60 mm; 

 length of the postannular region, c. 57 mm; overall length of the body, 

 c. 175 mm. 



* Ushakov (1933) described them as "curved chitinous bands". 



