In the Amphipoda, to which the above-mentioned naturalists correctly refer the Caprellidæ and Cyamidæ (Latreille’s _Læmodipoda_), the heart has always the same position; it extends in the form of a long tube through the six segments following the head, and has three pairs of fissures, furnished with valves, for the entrance of the blood, situated in the second, third, and fourth of these segments. It was found to be of this structure by La Valette in _Niphargus_ (_Gammarus puteanus_), and by Claus in _Phronima_; and I have found it to be the same in a considerable number of species belonging to the most different families.[1] The sole unimportant exception which I have hitherto met with is presented by the genus _Brachyscelus,_[2] in which the heart possesses only two pairs of fissures, as it extends forward only into the second body-segment, and is destitute of the pair of fissures situated in this segment in other forms.[3] Considering this uniformity presented by the heart in the entire order of the Amphipoda, it cannot but seem very remarkable, that in the very next order of the Isopoda, we find it to be one of the most changeable organs. In the cheliferous Isopods (_Tanais_) the heart resembles that of the Amphipoda in its elongated tubular form, as well as in the number and position of the fissures, but with this difference, that the two fissures of each pair do not lie directly opposite each other. Fig. 14. Heart of a young Cassidina. Fig. 15. Heart of a young Anilocra. Fig. 16. Abdomen of the male of Entoniscus Cancrorum. h. Heart. l. Liver. Fig. 14. Heart of a young _Cassidina._ Fig. 15. Heart of a young _Anilocra._ Fig. 16. Abdomen of the male of _Entoniscus Cancrorum. h._ Heart. _l._ Liver.