THE blood of the invertebrata, like that of the vertebrata, is not homogeneous. It consists of a transparent or semi-transparent liquid, and a number of small, solid corpuscles, which float in it.

In the higher animals the corpuscles are of two kinds-red and colourless; but in the invertebrata there are, as a rule, only colourless corpuscles. The red blood of annelids is different from the red blood of vertebrates, inasmuch as the plasma is coloured and the corpuscles are colourless in the former,* while in the latter the plasma is colourless, and there are present coloured and colourless corpuscles.


There are exceptions to this general statement.


The corpuscles in the blood of the invertebrata are of different sizes, and the size varies greatly in the same individual. Their form, however, is generally spherical, and their surface has a raspberry appearance.

In the higher invertebrata the blood clots after a variable period of time. Haycraft and Carlier* have examined the coagulation of the blood in certain invertebrates. According to their investigations the clot is formed, at any rate for the greater part, by the welding together of blood corpuscles. These throw out processes which interlace to form a solid


Although the blood of the invertebrata contains corpuscles its composition greatly varies. For instance, the blood of the lower, and some of the higher, invertebrates is a watery fluid containing proteids. This kind of blood has been termed hydrolymph. But in the majority of the higher invertebrates the blood is less watery, and consequently much richer in proteids. This variety of blood is called hæmolymph. It is generally stated that the distinction between these two varieties lies not only in their composition but also in the physiological functions which they perform.

*Haycraft and Carlier, Proc. Roy. Soc. Edin., vol. xv. p. 423.

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