segment three (3); the terminal setae on segment three 

 (2); and the outer setae on segment three (3). Therefore, 

 our setal formula would be: 



Enp. 

 1.2.321 



Exp. 



1.1.323 



When this is done for legs 1-4 and presented in tabular 

 form, it is a very quick way to compare these important 

 taxonomic characters. 



P5 is dimorphic and varies so widely that generaliza- 

 tion about it is difficult. However, in most cases the 

 coxa-basis and endopod are fused into a nonsegmented 

 platelike structure called the baseoendopod (Benp) to 

 which the nonsegmented plate or leaflike exopod (Exp) is 

 attached (there are a few genera in which the P5 exopod is 

 .segmented). Often however, the left and right baseoen- 

 dopods are fused together forming a continuous plate 

 across the entire ventral side of the body. The male append- 

 age is constructed similarly to that of the female but is 

 always much smaller and less ornate. The female P5 legs 

 are often used as a broad pouch or as protecting flaps for 

 the developing externally attached eggs (Fig. 9). 



The caudal rami, often misnamed the furcae (see Bow- 

 man 1971), are articulated to the last urosomal somite 

 and have at least one major seta (usually two setae) pro- 

 jecting posteriorly. The caudal rami are usually the same 

 in both sexes, but there are several genera which may 

 have dimorphic caudal rami (e.g., Phyllopodopsyllus, 

 Enhydrusuma). 



Ecology 



As in many other marine groups, the greatest number 

 of harpacticoids live in shallow-water sediments and/or 

 in the phytal zone. In the benthos harpacticoids are sec- 

 ond only to nematodes in overall abundance and in 

 some areas are often the most abundant taxon found in 

 the meiobenthos. Harpacticoids usually follow one of 

 three modes of existence in the sediment: 1) interstitial, 

 2) burrowing, or 3) epipelic (surface living) (see Fig. 1 for 

 various body shapes). The interstitial harpacticoids 

 (e.g., Cylindropsyllidae) are typically vermiform, 

 elongate animals that occupy the interstices of sands by 

 wriggling around and between the sand particles. The 

 burrowers are generally broadened at the cephalothorax 

 {Halectmosoma. Diosaccidae) for pushing sediment par- 

 ticles out of their path or are equipped with spade- 

 shaped appendages (Cerviniella, some Cletodidae) for 

 digging in the sediments. They are most common in fine 

 sediments with a median particle diameter below 2(X) 

 jjm, i.e., muds, silt-clays. The epipels are those harpac- 

 ticoids that typically live on the surface of the sediment 

 and, in many cases, are adapted morphologically to this 

 mode of existence by the great elongation of body limbs 

 [Malacopsyllus. Mesocletodes) to increase the surface to 

 volume ratio which, since they usually occur on soft 

 sediments (particularly in the deep sea), allows them to 

 walk over the surface of fluidlike mud without sinking. 



The phytal (or epiphytic) species are extremely com- 

 mon on marine algae and angiosperms, e.g., Zostera, 



BASEOENDOPODITE 



Ppid- 



CLETODES PSEUDODISSIMILIS COULL 



Figure 9.— The fifth leg (P > of a female and male Cletodes pseudodissimilU. Note the large size dif- 

 ferentiation between the two sexes. 



