Taken primarily from Kozloff, 1987, 1996 p.387 (Copyright 1987, 1996, University of Washington Press. Used in this web page by permission of University of Washington Press)
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| Lower taxonomic level | Main Page | Alphabetic Index | Systematic Index | Glossary |
| 1a | With gills on pereonites 2-4; mandible without molar process | 2 |
| 1b | With gills on pereonites 3 and 4 only; mandible with a molar process | 3 |
| 2a | Perepods 3 and 4 with 1 article, pereopod 4 with 1 article; abdomen small and without obvious segmentation | Cercops compactus |
| 2b | Pereopods 3 and 5 with 3 articles, pereopod 4 with 1 article; abdomen small and without obvious segmentation | Petrotripus brevis |
| 3a | With vestigial pereopods on pereonites 3 and 4; mandible with a palp | 4 |
| 3b | Without pereopods on pereonites 3 and 4; mandible without a palp | 7 |
| 4a | Pereopods 3 and 4 consisting of 2 articles; antenna 2 without long setae | 5 |
| 4b | Pereopods 3 and 4 consisting of only 1 article; antenna 2 with long setae on the larger articles | 6 |
| 5a | Head with a median dorsal spine (in the male) or a knob (in the female); pereopod 5 consisting of 6 articles; gnathopod 2 of male attached to the anterior half of pereonite 2 | Deutella californica |
| 5b | Head without a median dorsal spine or knob; pereopod 5 consisting of 3 articles; gnathopod 2 of male attached to the posterior half of pereonite 2 | Mayerella banksia |
| 6a | Next-to-last article of antenna 2 about 4 times as long as wide, with setae not appreciably longer than its width; short spines on pereonites directed more nearly anteriorly than laterally | Tritella laevis |
| 6b | Next-to-last article of antenna 2 about 6 times as long as wide, with some setae about twice as long as its width; short spines on pereonites directed more laterally than anteriorly | Tritella pilimana |
| 7a | Width of propodus of gnathopod 2 about four-fifths the length of this article; length of adults about 4 mm; with a pair of grasping spines on the proximal half of the propodus of pereopods 5-7 (found on the surface of the sea star Henricia leviuscula, as well as on hydroids and other organisms) | Caprella greenleyi |
| 7b | Width of propodus of gnathopod 2 less than two-thirds the length of this article; length of adults generally greater than 5 mm; with a single grasping spine or tooth on the propodus of pereopods 5-7 | 8 |
| 8a | With a ventral spine between the bases of gnathopods 2 | 9 |
| 8b | Without a ventral spine between the bases of gnathopods 2 | 12 |
| 9a | ||
| 9b | ||
| 10a | ||
| 10b | ||
| 11a | ||
| 11b | ||
| 12a | Head without any dorsal spines, knobs, or tubercles | 13 |
| 12b | Head with 1 or a pair of dorsal spines, knobs, or tubercles | 18 |
| 13a | ||
| 13b | ||
| 14a | ||
| 14b | ||
| 15a | ||
| 15b | ||
| 16a | ||
| 16b | ||
| 17a | ||
| 17b | ||
| 18a | Head with a single dorsal spine or knob | 19 |
| 18b | Head with a pair of dorsal spines or tubercles | 23 |
| 19a | Dorsal spine on head an anteriorly directed, triangular projection | 20 |
| 19b | Dorsal projection on head a knob that is directed upward | 22 |
| 20a | Tubercles on dorsal surface of pereonites small or absent (if present at all, lacking on pereonite 1); gnathopod 2 of male attached near the anterior end of pereonite 2 | Caprella angusta |
| 20b | Tubercles on dorsal surface of pereonites distinct, present on all pereonites; gnathopod 2 of male attached near the middle of pereonite 2 | 21 |
| 21a | ||
| 21b | ||
| 22a | ||
| 22b | ||
| 23a | ||
| 23b | ||
| 24a | ||
| 24b | ||
| 25a | ||
| 25b | ||
| 26a | ||
| 26b | ||
| 27a | ||
| 27b | ||
| 28a | ||
| 28b | ||
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| 29b | ||
| 30a | ||
| 30b | ||
| 31a | ||
| 31b | ||
| 32a | ||
| 32b | ||
| 33a | ||
| 33b | ||
| 34a | ||
| 34b | ||
| 35a | ||
| 35b | ||
Caprella angusta
Mayerella banksia
Tritella pilimana