248 THE LAMELLIBRANCHIA 
velum, at the base of the first internal branchial filament, but their 
persistence has not been demonstrated except in the Mytilidae and 
Avicula (Fig. 236, e). On either side of the pedal centres an 
ectodermic invagination gives rise to an otocyst; the invagina- 
tions close up in most cases, but remain open in Nucula and 
Mytilus. The apical portion of the velar area gives rise to the 
labial palps. (5) In all groups of the Lamellibranchia (Nuculidae, 
Mytilidae, Avicula, Ostraea, Fig. 192, 9, Dreissensia, Entovalva, Pisidium, 
Anadonta, Fig. 228, br, ete.), the branchiae originate in the form of 
filaments, which develop one by one from behind forwards, at the 
posterior part of the body, on the right and left sides, between the 
mantle and the visceral mass. The filaments of the internal gill- 
plate are the first to be formed, afterwards those of the external 
gill-plate : it is only at a late period that the filaments are reflected 
and unite with one another. (6) Two larval kidneys have been 
found in several groups (Dreissensia, Cyclas, Teredo), in the form of 
small organs of ectodermic origin, situated on either side of the 
anterior end of the larva, behind the velum, and opening to the 
exterior by their hinder ends (Fig. 224, re). Each organ consists 
of two cells (Dreissensia), of which one is deep and ciliated, the other 
is tubular, with an intra-cellular canal leading from the flagellum of 
the deeper cell to the external orifice. (7) In the trochosphere or 
veliger larva provided with a bivalve shell, the anterior adductor 
muscle is the first to be developed, as may be seen in Nucula, the 
Mytilidae, Ostraea, Pecten, Lasaca, Entovalva, Dreissensia, Pisidium, 
the Unionidae, Cardium, ete. In considering the evolution of the 
larva one must distinguish between two quite different modes 
of development, one of which may be called the normal mode, 
while the other is characterised by the parasitism of the larva 
and subsequent metamorphoses (Unionidae). In the first mode 
one may further distinguish a development through a veliger 
larva, which occurs in most Lamellibranchs, and a development 
through testaceous larvae, characteristic of the Nuculidae. In the 
development with a veliger stage, the larva may be free, as is 
the case in many marine forms and in the freshwater Drevssensia, 
or it may be retained and incubated in the gills, as in Cyclas, 
Kellya, Teredo, etc. When the larva leads a free existence its velum 
is always rather prominent (Fig. 224, v), but when it is retained 
and incubated by the parent the velum is reduced or sometimes 
disappears altogether (Cyclas, Unionidae, Hntovalva). When the 
velum is absorbed the foot becomes highly developed, even in 
such forms as become sedentary and fixed in after life, such as 
Pecten, Avicula, ete., unless indeed they attach themselves at a 
very early period. 
In the Nuculidae, which have test-larvae, we find that in 
Yoldia and Nucula provima the ova are set free in the water 
