[1] As this was not observed in the sea, but in glass vessels containing sea-water, it might be supposed that the animals become exhausted and die, not because they are under water but because they have consumed all the oxygen which it contained. I therefore put into the same water from which I had just taken an unconscious Ocypoda, with its legs hanging loosely down, a specimen of Lupea diacantha which had been reduced to the same state by being kept in the air, and this recovered in the water just as the Ocypoda did in the air.
[2] This smaller Gelasimus is also remarkable because the chameleon-like change of colour exhibited by many Crabs occurs very strikingly in it. The carapace of a male which I have now before me shone with a dazzling white in its hinder parts five minutes since when I captured it, at present it shows a dull gray tint at the same place.
CHAPTER VI.
STRUCTURE OF THE HEART IN THE EDRIOPHTHALMA.
Scarcely less striking than the example of the air-breathing Crabs, is the behaviour of the heart in the great section Edriophthalma, which may advantageously be divided, after the example of Dana and Spence Bate, only into two orders, the Amphipoda and the Isopoda.
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.