. Bulletin - United States National Museum. Science. vol. 2, pt. 2.] A TAXONOMIC STUDY OF THE SALPIDAE METCALF. 11 series of increasingly developed eyes. The character of the rod cells in the imperfect eyes of the more aberrant Salpidae seems clearly to indicate degeneration. A typical rod cell from any well-developed Salpa eye would be about as shown in figure 1. Compare this with a group of rod cells from, say, Pegea confed erata (fig. 2). In the former the glassy mod- ification of the protoplasm, which constitutes the "rod" (represented in black in the figures), is regular and is
. Bulletin - United States National Museum. Science. vol. 2, pt. 2.] A TAXONOMIC STUDY OF THE SALPIDAE METCALF. 11 series of increasingly developed eyes. The character of the rod cells in the imperfect eyes of the more aberrant Salpidae seems clearly to indicate degeneration. A typical rod cell from any well-developed Salpa eye would be about as shown in figure 1. Compare this with a group of rod cells from, say, Pegea confed erata (fig. 2). In the former the glassy mod- ification of the protoplasm, which constitutes the "rod" (represented in black in the figures), is regular and is found at one end of the cell, as is characteristic of the rod cells of the eyes of vertebrates, of Amphioxus, of the ascidian tadpole, and of most invertebrates. On the other hand, in those species of Salpidae whose eyes I would consider degenerate, the rod cells are very irregular in shape, are irregularly crowd- ed into masses instead of being arranged as a reg- ular epithelium, and each rod cell has the pro- toplasm usually of its whole periphery con- verted into the typical glassy substance, this fig. i.—a schematic repre- glassy layer being of different thickness in dif- ll^^ZtsmGLE ROD ° J J & . CELL AND FOUR PIGMENT CELLS ferent rod cells and in different portions of the from the eye of a salpa. same cell. FK0M METCALF (1893'c)- Taking, then, as our starting point Cyclosalpa pinnata, which, from the arrangement of its muscles and-the structure of its eyes, seems one of the two most primitive of the species of Salpidae, let us note its general structure, giving attention to a few new features we have discovered. Then let us briefly review the anatomy and development of its eyes, as already described by Metcalf (1893, c). Having refreshed our memories of the salient features in this species, we can proceed to comparisons with other species. Cyclosalpa pinnata is represented in the collec- tions of the United States National Museum by the following catalogue numbers:
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