. Comparative anatomy and physiology. Chap. VI.] TRACHEA. 215 has directed attention to the deeper slits and larger amount of haemoglobin in the Schizonemertini, as being correlated with their habit of dwelling in mud and other places in which the supply of oxygen is small; the ciliated cells of the groove clearly serve to drive in currents of sea-water. In c and D (Fig. 92) it will be observed that the lower part of the ganglionic mass is shaded more lightly than the rest; the cells that form this portion are derived from the oesophagus, from the walls of which, however, they subsequently bec
. Comparative anatomy and physiology. Chap. VI.] TRACHEA. 215 has directed attention to the deeper slits and larger amount of haemoglobin in the Schizonemertini, as being correlated with their habit of dwelling in mud and other places in which the supply of oxygen is small; the ciliated cells of the groove clearly serve to drive in currents of sea-water. In c and D (Fig. 92) it will be observed that the lower part of the ganglionic mass is shaded more lightly than the rest; the cells that form this portion are derived from the oesophagus, from the walls of which, however, they subsequently become separated; and we observe, therefore, that an out-pushing from the gullet goes to meet the epiblastic in-pushing from the surface. An essentially similar phenomenon is to be observed in the Eiitero- piieusti (Balano- glossus) and in the Cliordata. (See page 231.) In the traclie- ate Artliropoda we have examples of in-pushings of the surface adapted for the entrance not of water, but of air ; they are seen at their simplest in Peripatus, where they are distributed over the whole of the body. The tracheal orifice leads into a pit, which traverses the dermis, and widens out at its inner end ; from this the tracheae arise as minute tubes, which rarely branch, and only have a faint indication of the presence of the spiral fibre, which, in higher Tracheates, gives so. Fig. 93.—Trachea? of Insect, showing the Spiral Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Bell, F. J. (Francis Jeffrey), 1855-1924. London, Cassell
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