316 A HISTORY OF RECENT CRUSTACEA 



perason. The six segments of the pleon may be all 

 articulated, or some or all of them fused together. The 

 seventh segment or telson is scarcely ever free, but its in- 

 dependent existence is indicated in the peculiar genus 

 Phreatoicus, and in one tribe, the Flabellifera, its presence 

 may generally be inferred from the attachment of the 

 uropods high up on the sides of the terminal segment, 

 while a few of the genera, as Paranthura, have it distinctly 

 articulated. 



A pair of sessile compound eyes, remote or contiguous, 

 are usually present, but may be entirely wanting. Occa- 

 sionally they project from the head, but never on movable 

 foot-stalks. 



The first antennas are never very elongate, and rarely 

 have a secondary flagelluni. The second antennas very 

 seldom carry an exopod, the articulated scale so common 

 among the Macrura. The upper lip usually forms a plate 

 projecting from the top of the oral aperture over the cutting 

 edges of the mandibles, and may have an inner plate lying 

 parallel to the outer. The lower lip is bilobed, or forms two 

 pairs of lobes, of which the inner pair is much the smaller. 

 The mandibles are very variable, the dentate cutting edge, 

 secondary plate, spine-row, molar tubercle, and three-jointed 

 ' palp,' being sometimes strongly developed, at others, some 

 or even all of them disappearing. The first maxillae very 

 rarely have a backward directed ' palp.' The second 

 maxillaa have neither ' palp ' nor exopod ; the outer plate 

 is divided, the inner undivided. The maxillipeds, of which 

 the first are in this and the next sub-order the only pair, 

 often have an epipod on the first joint. This first joint as 

 a rule stands free in each maxilliped, the pair not being 

 fused together. The plate developed from, the second 

 joint is provided with coupling spines. For the limbs of 

 the trunk it is difficult to find any characters that are at 

 all constant through the sub-order. Dr. Boas in his defi- 

 nition (1883) says that these seven pairs are strong running- 

 feet with a spine at the point of the terminal joint ; that 

 the basal joint is always small, the second joint long, the 

 third in the Tanaidas smaller than those which follow, 



