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,Y\g. 192, g, Dreissensia, Entovalva, Pisidium, 

 Anadonta, Fig. 228, br, etc.), 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, Cydas, 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, Lasaea, Entovalva, Dreissensia, Pisidium, 

 the Unionidae, Cardium, etc. 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 Dreissensia, 

 or it may be retained and incubated in the gills, as in Cydas, 

 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 (Cydas, Unionidae, Entovalva). 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, etc., unless indeed they attach themselves at a 

 very early period. 



In the Nuculidae, which have test-larvae, we find that in 

 Yoldia and Nucula proxima the ova are set free in the water 



