In all other Isopoda the heart is removed towards the abdomen. In the wonderfully deformed parasitic Isopods of the _Porcellanæ_ (_Entoniscus porcellanæ_), the spherical heart of the female is confined to a short space of the elongated first abdominal segment, and seems to possess only a single pair of fissures. In the male of _Entoniscus Cancrorum_ (n. sp.), the heart (Fig. 16) is situated in the third abdominal segment. In the _Cassidinæ,_ the heart (Fig. 14) is likewise short and furnished with two pairs of fissures, situated in the last segment of the thorax and the first segment of the abdomen. Lastly, in a young _Anilocra,_ I find the heart (Fig. 15) extending through the whole length of the abdomen and furnished with four (or five?) fissures, which are not placed in pairs but alternately to the right and left in successive segments. In other animals of this order, which I have as yet only cursorily examined, further differences will no doubt occur. But why, in two orders so nearly allied to each other, should we find in the one such a constancy, in the other such a variability, of the same highly important organ? From the schoolmen we need expect no explanation, they will either decline the discussion of the “wherefore” as foreign to their province, as lying beyond the boundaries of Natural History, or seek to put down the importunate question by means of a sounding paraphrase of the facts, abundantly sprinkled with Greek words. As I have unfortunately forgotten my Greek, the second way out of the difficulty is closed to me; but as I luckily reckon myself not amongst the incorporated masters, but, to use Baron von Liebig’s expression, amongst the “promenaders on the outskirts of Natural History,” this affected hesitation of the schoolmen cannot dissuade me from seeking an answer, which indeed presents itself most naturally from Darwin’s point of view.