[4] Whether the want of the abdominal feet in the young of _Tanais_ be an inheritance from the time of the primitive Isopoda, or a subsequently acquired peculiarity, which appears to me the more admissible view at present, may perhaps be decided with some certainty, when we become acquainted with the development and mode of life of its family allies, _Apseudes_ and _Rhœa._ The latter, as is well known, is the only Isopod which possesses a secondary flagellum on the anterior antennæ. I have recently obtained a new and unexpected proof that the _Tanaidæ_ (“Asellotes hétéropodes” M.-Edw.) of all known Crustacea approach most closely to the primitive form of the Edriophthalma. Mr. C. Spence Bate writes to me: “_Apseudes,_ as far as I know, is the _only_ Isopod in which the antennal scale so common in the Macrura is present on the lower antenna.” INDEX. (Numbers represent chapter number. Click on number and use 'Find' in the 'Edit' menu of your browser. Numbers in parentheses denote number of references in the chapter.) Acanthonotus Owenii, 2. Acanthosoma, 7. Achæus, 7 (2). Achtheres, 12. —— percarum, 9 (2). Allorchestes, 4, 8. Alpheus, 6, 7, 12. Amphilochus, 2, 8. Amphipoda, 3, 6, 8 (2), 12. Amphithoë, 2, 8. Anceus, 8. Anelasma squalicola, 12. Anilocra, 6. Aratus, 2. —— Pisonii, 5. Artemia, 9. Asellus, 8. Atylus, 8. —— carinatus, 2. Batea, 8. Bodotria, 4, 8. Bopyridæ, 8 (2). Bopyrus, 8 (2). Brachyscelus, 6, 8 (2), 9, 10 (2), 12. —— crusculum, 8. Brachyura, 12. Branchiopoda, 9. Calanidæ, 10. Caligus, 8. Caprella, 8 (3). —— attenuata, 6. —— linearis, 6. Carcinus mænas, 7. Caridina, 7. Cassidina, 6, 8 (3). Cerapus, 2, 8 (3). Chalimus, 8. Chondracanthus, 9. Chthamalus, 9. Cirripedia, 9 (2), 10. Cladocera, 9. Copepoda, 4, 9 (2), 10, 12. Corophium, 8 (2). —— dentatum, 8. Corycæidæ, 10. Crangon, 7. _Crayfish_, 10. Cryptoniscus planarioïdes, 8 (2). Cryptophialus, 12. —— minutus, 9. Cuma, 8. Cumacea, 8. Cyclograpsus, 4, 5, 7. Cyclopidæ, 10. Cyclops, 9 (2), 10. Cyclopsine, 9. Cymothoa, 8. _Cymothoadiens_, 8. Cypridina, 10. Cypris, 8, 10. Cyrtophium, 2, 8, 10. Cythere, 10. Daphnia pulex, 8. Dercothoë, 8. Diastylidæ, 4, 8. Dichelaspis Warwickii, 12. Dulichia, 8 (2), 10. Edriophthalma, 3, 6, 8. Entomostraca, 9. Entoniscus, 8. —— cancrorum, 6, 8 (2). —— porcellanæ, 6, 8 (2), 12. Erichthus, 7. Eriphia gonagra, 2, 5. Euphausia, 7 (2), 10, 12 (2). Eurynome, 7. Evadne, 8. Filograna, 11. Gammarus, 8. —— ambulans, 8. —— Dugesii, 4. —— puteanus, 6. Gecarcinus, 7 (2). Gelasimus, 2, 4 (2), 5, 7. —— vocans, 5. Glaucothoë Peronii, 7. Grapsus, 5 (2). _Hermit Crabs_, 7 (3), 12. Hippa emerita, 7 (2). Hippolyte, 7 (2), 12. Hyperia galba, 8. —— Latreillei, 8. —— Martinezii, 8. “_Hypérines anormales et ordinaires,_” 6, 8. Idothea, 8 (2). Insecta, 11. Isopoda, 3, 6 (2), 8, 12. Kepone, 8.