Mytilus edulis L. is the common, blue marine mussel found in the intertidal regions of northeast North America. It is a bivalve, plecypod mollusk that in its adult form attaches itself to rocks and pilings by a network of tough byssus threads, which it spins. (Byssus is a natural biopolymer spun by the animal for the express purpose of attaching itself to a supporting substrate.) The eyes of Mytilus lack any dioptric apparatus (a lens or pinhole) with which to form an image on its primitive photo-receptor cell array. Mytilus has two eyes; each eye is a simple invaginated cup, containing about 37 photoreceptor cells (PRCs), and is about 50 to 60 ^m in diameter, regardless of animal size. Figure 2.8-1 shows a light micrograph of a transverse section through an eye. A larger magni cation of an e ye is shown in Figure 2.8-2. Note the granules in the pigment cells. The eyes are located at the base of the left and right inner gill laments, between the inner and outer gills. Each PRC sends an axon directly in the optic nerve to the cerebral ganglion. LaCourse (1981), observed that there was one optic nerve ber per PRC. That is, this simple visual system has no intervening synapses between the PRCs and the optic nerve bers.
FIGURE 2.8-1 Artist's line drawing of a transverse section through an eye of M. edulis (from a light micrograph). Object in the eyecup is an artifact; a shrunken ball of mucus and cilia. Scale line is 20 <*=m. (From LaCourse, J.R., Ph.D. dissertation, University of Connecticut, Storrs, 1981. With permission.)
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