. Discovery reports. Discovery (Ship); Scientific expeditions. INTERNAL ANATOMY 249 each side of the pharynx as a band of vacuolated cells with a deep internal groove com- municating with the cavity of the pharynx. The insertion of these lateral bands, the pleurochords, divides the wall of the pharynx more or less completely into four distinct regions, two lateral, a mid-dorsal and a mid-ventral. The two lateral regions are formed of the bands of vacuolated cells, and the mid-dorsal and mid-ventral walls of ordinary cells of the pharynx. The groove on the inner side of the pleurochords is deep
. Discovery reports. Discovery (Ship); Scientific expeditions. INTERNAL ANATOMY 249 each side of the pharynx as a band of vacuolated cells with a deep internal groove com- municating with the cavity of the pharynx. The insertion of these lateral bands, the pleurochords, divides the wall of the pharynx more or less completely into four distinct regions, two lateral, a mid-dorsal and a mid-ventral. The two lateral regions are formed of the bands of vacuolated cells, and the mid-dorsal and mid-ventral walls of ordinary cells of the pharynx. The groove on the inner side of the pleurochords is deep and conspicuous in C. hodgsoni, and, as already mentioned, in C. densiis a tubular space com- pletely lined with vacuolated cells lies between the external gill opening and a cavity of the pharynx. This has been termed the gill cavity, and its opening into the pharynx the internal gill slit. The respiratory water passes through the gill cavity before it finds outlet through the external gill slit. This gill cavity could have been formed only by the fusion of the lips of the groove in the pleurochords. In other species where the lips of the groove stand apart, the respiratory water probably traverses the groove before passing through the gill slit. Assuming this to be so, it is clear that the pleurochords and the grooves play a definite part in respiration. Further, the pharyngeal blood vessels lead to the vacuolated region of the roof of the gill slit, but the mode of exchange of gases still remains a problem. A rod-shaped structure with a narrow lumen projects upwards from the dorsal wall of the anterior diverticulum of the pharynx. This organ has been variously termed by diff'erent writers. Harmer (1887, p. 40) compared it with the notochord of Balanoglossiis. Later, Willey (1902) de- scribed it as the stomatochord, and Masterman (1898 h, p. 351) regarded it as the subneural gland; but the view most generally accepted was that this organ corresponds to the vermiform pro- ces
Size: 1145px × 2183px
Photo credit: © Paul Fearn / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1920, booksubjectscientificexpedition
