(1) The blood of Carcinus masnas exhibits three chief conditions--colourless, pink, and yellow--the pink and yellow colour being due to the two lipochrome pigments, tetron-erythrin and lutein. The pink colour appears in individuals, especially in males, which are approaching the period of a moult, the yellow is characteristic of the female when the ovary is approaching maturity.

(2) An estimation of the fat content of the blood, by means of saponification and extraction of the fatty acids, gives the following average nnmbers: for colourless blood .059 per cent., for pink blood of males .086 per cent., for yellow blood of females .198 per cent. Thus the breeding females are shown to possess an excess of fatty material in tlie blood, and the yellow lipochrome is seen to represent a higher fat value than the pink.

(3) Beside the blood, the " liver " also exhibits periodic variations in the amount of fat present, fat being sometimes present us 12 per cent, of the total weight, sometimes as low as 4 per cent. The females maturing their ovary and having yellow blood always have a large proportion of fat in the liver.

(4) Crabs of both sexes infected with Sacculina also always show a large supply of fat in the "liver." In the case of Carcinus the blood of infected individuals is either colourless or pale yellow, but in the case of Inachus the blood becomes charged with lipochrome, as the result of infection, though this lipochrome always shows the presence of the pink colour as well as the yellow.

(5) The difference in reaction of the blood in the two cases is explained as consistent with the small effect which Sacculina exerts on all the characters of Carcinus compared with its effects on Inaclius.

(6) The investigation shows that Sacculina exerts a marked influence on the fat-inetabolisra of the host, and the results are on the whole consistent with the view that Sacculina influences the host to assume the female characters by acting the same part in the fat-metabolism as the ripening ovary does in a normal female.

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