. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. Development of Dreissena polymorphs, Pallas. 163 siderably increased in size, and already presents the appear- ance of the bivalve mussel-shell (fig. 1). This condition of the larva, apart from the processes which take place in the interior, continues for a long time. As it is peculiarly charac- teristic, and is useful for the discovery of the larva, a few- sketches of it are given herewith. Fig. Younger larva of Dreissena, with bivalve shell (s) and velum (vel), seen from
. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. Development of Dreissena polymorphs, Pallas. 163 siderably increased in size, and already presents the appear- ance of the bivalve mussel-shell (fig. 1). This condition of the larva, apart from the processes which take place in the interior, continues for a long time. As it is peculiarly charac- teristic, and is useful for the discovery of the larva, a few- sketches of it are given herewith. Fig. Younger larva of Dreissena, with bivalve shell (s) and velum (vel), seen from the side, a, amis; m, stomach; mu, mouth; sm, adductor muscle. Before all things important and characteristic is, besides the shell, which is at first delicate and composed of a thin cuticular membrane, and subsequently somewhat stouter, the extensive velum. The velum appears as, so to speak, a fleshy organ, which is beset with stout cilia at its margin. It possesses a peculiar pigmentation, which greatly increases with the age of the larva (figs. 1 and 2). Fig. 1 shows the velum as seen from the side in a younger larva, and the following figures exhibit it in older larvae in different positions. From fig. 3, which represents an older larva seen from the side, it is evident what a considerable size the velum may assume. The larva is usually observed as represented in fig. 2. It swims on the surface of the water, with the velum directed upwards. In front and behind we see the valves of the shell projecting beneath the expanded velum (figs. 2, 3). The ciliary movement is most readily comparable to the rotation of a circular saw, since only a certain number of the cilia are extended simultaneously, while the interjacent ones appear to be bant. Sometimes the whole of the cilia are stretched straight out. The motion of the larva is very rapid, and 11*. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - colora
Size: 1713px × 1459px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookce, booksubjectbotany, booksubjectgeology, booksubjectzoology
