170 PART II. SYSTEMATIC ACCOUNT 



The protosoma is two-fifths of the length of the mesosoma, getting some- 

 what narrower towards the front and rimmed at the back by an edging which 

 takes the form of a circular cutaneous fold (Fig. 93А У В, C). The conical 

 cephalic lobe is somewhat flattened dorso-ventrally and limited behind by a 

 slight ridge. The tentacular crown consists of 12 tentacles, and not of 13, as 

 was incorrectly stated earlier (Ivanov, 1952), and they are attached in the form 

 of a horseshoe, with the ends drawn together ventrally. On the inner surface 

 of each tentacle the short pinnules are arranged in two alternating rows. 



Along the ventral side of the protosoma and mesosoma runs a sharply 

 marked median groove, or furrow, which closes to a narrow slit in front of 

 the bridle, and then opens out again on the protosoma where the edges fork 

 to either side. The keels of the bridle are very delicate and joined to each 

 other ventrally (Fig. 93Д В, C). On the ventral side it can readily be 

 observed that the hind edge of the mesosoma is bilobed and overhangs the 

 deep groove which divides it from the trunk (Fig. 93B). 



The metameric part of the trunk is dorso-ventrally compressed (Fig. 

 93A, C, F), and the ridges bounding the ventral sulcus are moved rather far 

 onto the sides of the body, thus appearing almost lateral, instead of ventral, 

 and enhancing the flattening of this region. The papillae of the metameric 

 region are rather sharply detached and delimited from one another, and the 

 front ones are relatively short, becoming larger only farther back. In con- 

 sequence of the lateral position of the ridges and papillae which bear them, 

 pulvilli with cuticular plaques often lie on the side of the body (Fig. 93F). 

 The total number of metameric papillae is 50-55 pairs and they contain 

 numerous pyriform glands which are readily visible through the integument. 

 A wide dorsal ciliated organ runs along the length of the metameric region, 

 ending at the level of the 45 th to 47th pair of papillae. The genital papillae of 

 the males are oval, and approach one another at the front end, where they 

 overhang the hind part of the mesosoma (Fig. 93D). The plaques are small 

 and oval, measuring about 20^ across, and have crescentic thickenings along 



Fig. 93. Birsteinia vitjasi: A - front part of body of a male from the right side; В - the same in 

 ventral view; С - the same in dorsal view; D - hind part of the mesosoma and front part of the 

 metasoma of a male in ventral view; E- cuticular plaques from the metameric part of the pre- 

 annular region; F - metameric part of the preannular region in ventral view; G - two metameric 

 papillae from one side. 



av - ridge beside ventral sulcus ; с - cephalic lobe ; со - dorsal ciliated band ; /- keel of the bridle ; 

 go - genital papilla ; g t - pyriform gland seen by transparency ; ms - mesosoma ; mts - metasoma ; 

 p - cuticular plaque ; pa - papillae ; pi - pinnule ; ps - protosoma ; pu - pulvilli ; s - median ventral 

 furrow ; x 1 - groove between the protosoma and the mesosoma ; s 2 - groove between the mesosoma 

 and the metasoma; sv - ventral sulcus; t - tentacle. (A, B, D, E, F- after Ivanov, 1952; A and В 

 somewhat modified.) 



