. A manual of zoology. Zoology. I. AMPHINEURA 315 veliger arises from the velum, a strong circle of cilia, which surrounds a velar field in front of the mouth, and which serves as a locomotor organ for the larva. In some cases (fig. 314, B) it is lobed like the trochus of a Rotifer. The veliger recalls the annelid trochophore and serves for the distribution of the species; it is therefore of great importance for animals. Fig. 314.—Veliger stages, .1, of a snail; B, of a Pteropod (from Gegenbaur). 0, shell; op, operculum; p, foot; t, tentacle; v, velum. which, like most molluscs, are sedentary
. A manual of zoology. Zoology. I. AMPHINEURA 315 veliger arises from the velum, a strong circle of cilia, which surrounds a velar field in front of the mouth, and which serves as a locomotor organ for the larva. In some cases (fig. 314, B) it is lobed like the trochus of a Rotifer. The veliger recalls the annelid trochophore and serves for the distribution of the species; it is therefore of great importance for animals. Fig. 314.—Veliger stages, .1, of a snail; B, of a Pteropod (from Gegenbaur). 0, shell; op, operculum; p, foot; t, tentacle; v, velum. which, like most molluscs, are sedentary or slow-moving. In cases with- out metamorphosis (Cephalopoda, Pulmonata, etc.) the veliger stage is frecjuently indicated during embryonic development by a ridge of cells surrounding a preoral velar field Class I. Amphineura. These forms, some of which appear in the Silurian, are clearly the most primitive of molluscs, and are distinguished by a marked bilateral sym- metry. The nervous system already described (p. 3 13) consists of pleural and pedal cords with scattered ganglion cells and no ganglia, these cords being connected by numerous commissures (fig. 315, B). Sub Class I. Placophora {Chitonida:). The chitons were formerly included among the gasteropods because of the presence of a creeping foot and a radula. They are at a glance distinguished from them by the rudimentary condition of the head (which lacks tentacles and eyes), and the peculiar shell. This last consists of eight transverse plates, overlapping like shingles which allows the animal to roll into a ball. The edge of the mantle extends beyond the shell and is covered with spines, while in the mantle cavity beneath are, right and left, a series of ctenidia. Nerves enter the shell and end with noticeable sense organs {cesthetes and, in some, eyes, fig. 316). There are no stato- cysts. The symmetry of the body is also expressed in the Please note that these images are extracted from scanned page image
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
