SIBOGLINUM ARABICUM 291 



transverse groove can be observed in front of the bridle, that is to say, there 

 is no external boundary between the protosoma and mesosoma. The proto- 

 soma runs imperceptibly into the conical cephalic lobe which is pointed at 

 the tip (Fig. FF133.4). The tentacle is attached at the base of the cephalic 

 lobe. It is long and rather delicate and along its whole length I can find no 

 trace of pinnules. Its surface is smooth (Fig. FF133.4). 



The keels of the bridle are very delicate and hardly noticeable. Both 

 dorsally and ventrally their ends taper and are broadly separated (Fig. 

 FF133./4). Two smaller triangular glandular patches lie in front of the 

 bridle on the ventral side. The longitudinal glandular ribbons of the meso- 

 soma are well developed. They begin on the lateral surface of the mesosoma 

 where they run obliquely back, parallel to the keels of the bridle. Then they 

 approach one another on the ventral side and run back parallel to one 

 another to the hind end of the mesosoma, gradually tapering as they do so. 

 At the hind end the glandular ribbons break up into a series of small areas, 

 which probably consist of separate gland cells (Fig. FF133./4). The ribbons 

 do not reach as far as the front edge of the metasoma. In reflected light they 

 appear whitish. Oblique muscle fibres of the mesosoma can be distinguished 

 between them through the integument. 



The metameric part of the trunk is rather narrower than the mesosoma. 

 The ventral sulcus is rather deep and the paired papillae on either side of it 

 are crowded so tightly together that they almost merge into one another. 

 Each of them encloses a single clear pyriform gland (Fig. FF133y4). To- 

 wards the hind end of the metameric region the papillae become larger. 

 In each row they number about 65 in all. Alongside each row, on the lateral 

 surface of the trunk, extends a very broad glandular tract which is whitish 

 in reflected and opaque in transmitted light (Fig. FF133^4). The non- 

 metameric region has numerous irregular accumulations of large protuberant 

 gland cells containing a granular secretion (Fig. FF133B -g). 



There are two girdles, both widely broken on the ventral side. Their 

 protuberant ridges bear double rows of toothed platelets, 12-13/x long 

 (Fig. FF133C, D). Large plaqueless ventral papillae lie immediately in 

 front of and behind the girdles. The waist between the preannular and post- 

 annular regions of the trunk is represented by a deep transverse groove on 

 the ventral side (Fig. FF133C). The toothed platelets are bottle-shaped. 

 Their front section, bearing the anterior group of teeth, is narrow like the 

 neck of a bottle; it is only a third the width and a quarter the length of the 

 rest of the platelet (Fig. FF133Z), E). 



The postannular region is thin and probably long, and plaqueless conical 



