4 
AMPHIPODA. 
The animal is naturally divided into three parts : the 
head (or cephalon, c), formed of a single segment ; * the 
body (or pereion), consisting of seven segments (h to o) ; 
and the tail (or pleon), formed of six segments (p to v), 
exclusive of the terminal scale (or telson, z). These 
divisions are distinctly visible, and never encroach upon 
each other ; while the appendages assume characteristic 
forms in each division. Those which belong to the head 
are more or less connected with the organs of sense. 
The eyes (a) are sessile and compound. Their normal 
position is between the bases of the superior and inferior 
antennas. In the Orchestiidae and near allies they are on 
the top of the head, to which position they are thrust by 
the great increase of the size of the two basal articula- 
tions of the antennae and their absorption into the ante- 
rior portion of the head. The outer integument of the 
eyes is never divided into facets, except in some genera 
of the Hyperina. In many of the Phoxides the eyes 
appear to be wanting ; but this is probably caused by the 
absence of any colouring pigment, or its dispersion after 
death, rather than from the absence of the organ of 
vision. In Ampelisca they appear like four simple 
organs, resembling the ocelli of true insects. 
The anterior or superior pair of antennae ( b ) are formed 
* Adopting the theory of Oken, that each pair of limbs or organs indicates 
a separate segment (often, indeed, coalescing with the adjacent one), the head 
would consist of nine segments, namely : — 1st, that supporting the eye ; 2nd, 
the upper antennae ; 3rd, the lower antennas ; 4th, the upper lip, formed of 
two lateral halves united ; 5th, the mandibles, or jaws ; 6th, the lower 
lip, formed like the upper lip ; 7th, the first pair of maxillae ; 8th, the 
second pair of maxillae ; and 9th, the foot-jaws. If to these are added the 
seven segments of the body, the six segments of the tail, and the segment 
represented by the terminal scale, we have twenty-three segments as the 
normal number in the Ampliipoda. As, however, Mr. Spence Bate regards 
the two lips merely as the calcified extremities of the alimentary canal, the 
number of head-segments would be reduced to seven, and the entire number 
to twenty-one. — (I. 0. W.) 
