=Larvae of Unionidae.=--The early stages of the development of _Unio_ and _Anodonta_ (so far as the species of North America, Europe, and Asia are concerned) is of extreme interest, from the remarkable fact that the young live for some time parasitically attached to certain species of fresh-water fishes. In order to secure this attachment, the larva, which is generally known as _Glochidium_, develops a long filament which perhaps renders it aware of the neighbourhood of a fish, and also a larval shell furnished with strong hooks by which it fastens itself to the body of its unconscious host (Fig. 56). According to some interesting observations made by Mr. O. H. Latter,[264] the ova pass into the external gill of the mother, in which is secreted a nutritive mucus on which they are sustained until they arrive at maturity and a suitable opportunity occurs for their ‘being born.’ If this opportunity is deferred, and the _Glochidia_ mature, their so-called ‘byssus’ becomes developed, and by being entangled in the gill filaments of the parent, prevents their escaping. It is interesting to notice that, when the nutritive mucus of the parent is used up, it becomes, as it were, the turn of the children to provide for themselves a secondary mode of attachment. [Illustration: FIG. 56.--=A=, _Glochidium_ immediately after it is hatched: _ad_, adductor muscle; _by_, ‘byssus’ cord; _s_, sense organs; _sh_, shell. =B=, _Glochidium_ after it has been on the fish for some weeks: _a.ad_, _p.ad_, anterior and posterior adductors; _al_, alimentary canal; _au.v_, auditory vesicle; _br_, branchiae; _f_, foot; _mt_, mantle. (Balfour.)] The mother _Anodonta_ does not always retain the _Glochidium_ until fish are in her neighbourhood. Gentle stirring of the water caused them to emit _Glochidium_ in large masses, if the movement was not so violent as to cause alarm. The long slimy masses of _Glochidium_ were observed to be drawn back again within the shell of the mother, even after they had been ejected to a distance of 2 or 3 inches.