674 
J. DAVIDSON. 
The Endoskeleton of the Head (P]. 38, fig. 3 ; PI. 39, 
figs. 6-24; PI. 40, figs. 2-5-32). — The chitinous endoskeleton 
of tlie head consists of an arrangement of chitinous rods 
or endosternites, which, in addition to giving support 
to the head, afford attachment for muscles of the head 
and its appendag’es. Tlie endoskeleton in S. lanigera 
agrees closely with that described by Dreyfus (1894) in 
Phylloxera and Mordwilko (1895) in Lachnus. 
Situated in the posterior portion of the head, beneath the 
supra-oesophag'eal ganglion, is a hollow, chitinous plate 
or bar {t.), which forms the central support of the endo- 
skeleton. This structure corresponds to the “ Chitinstab ” of 
Mordwilko (1895) in Lachnus, named after Witlaczil 
(1882). It is also the arcus superior” of Mark (1877), 
Krassilstschik (1892-3), and other authors. Grove (1909) 
suggested the term “ transverse bar” for the corresponding 
structure in Siphonophora rosarum, and throughout my 
description of S. lanigera 1 shall use this term. 
Four hollow rods of chitin are fused with the transverse 
bar and pass from it to the walls of the head. The}’ are as 
follows : 
A pair of antero-dorsal rods (a. d.) (the Arcus inferiores) of 
j\lark (1877), Dreyfus (1894) and Mordwilko (1895), pass 
dorsally, one from each end of the transverse bar in an 
antero-lateral direction to the roof of the head, joining it at 
the junction of the clypeus and epicranium. Prom here a 
ridge of chitin (costas inferioies of Mark, Dreyfus, etc.) 
passes on each side of the clypeus along its lateral walls, with 
which they fuse. 
A pair of antero-ventral rods («. v.) embracing the arcus 
superiores and costte superiores of Dreyfus and Mordwilko 
pass ventrally from the ends of the transverse bar, in an 
antero-lateral direction, towards the infra-posterior border of 
the head, each being then reflected along the lateral walls of 
the clypeus to a position opposite the swollen proximal ends 
of the mandibles, where they fuse with the wall of the 
head. From this region, on each side, a stout chitinous 
