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 

 antennae. 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 (U] 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 antennae ; 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 Ampldpoda. As, however, Mr. Spence Bate regards 

 the two lips merely as the calcined 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.) 



