. The Cambridge natural history. Zoology. STRUCTURE BRANCHIAL SAC 47 of the branchial sac to the oesophagus, guided by a membranous fold, the dorsal lamina (Fig. 21, ), which is more or less ridged or corrugated, and may be armed with marginal tags or even replaced by larger processes (the " languets") in some species of Ascidians. In the living animal the lamina has its free edge curved to the right hand side in such a manner as to constitute a fairly perfect tube along which the train of food passes. Branchial Sac.—Thus we have the dorsal lamina (or the languets) along the dorsa
. The Cambridge natural history. Zoology. STRUCTURE BRANCHIAL SAC 47 of the branchial sac to the oesophagus, guided by a membranous fold, the dorsal lamina (Fig. 21, ), which is more or less ridged or corrugated, and may be armed with marginal tags or even replaced by larger processes (the " languets") in some species of Ascidians. In the living animal the lamina has its free edge curved to the right hand side in such a manner as to constitute a fairly perfect tube along which the train of food passes. Branchial Sac.—Thus we have the dorsal lamina (or the languets) along the dorsal edge, the endostyle along the ventral edge, and the peripharyngeal bands around the anterior end. The wall of the branchial sac itself is penetrated by a large number of channels through which blood flows. Some of these run in one direc- tion and some in an- other, so as to form complicated networks, which differ greatly in their arrangement in different Ascidians. Be- tween these blood- channels there are clefts (" stigmata "), the secondary or subdivided gill-slits, by means of which the current of water passes from the branchial sac to the large external peribran- chial or atrial cavity. ggs;^g-""'"~ > Fig. 21.—Antero-dorsal part of pharynx in Ascidia- mentula, x 15. Part of branchial sac ; , dorsal lamina ; , dorsal tubercle ; , pre- branchial zone ; , peripharyngeal bands ; sph, sphincter of branchial aperture ; tn, tentacle. All the stigmata (of which there may be several hundred thousand) in the wall of the branchial sac are bounded by cubical or columnar epithelial cells, which are ciliated. These cilia, so long as the animal is alive, are in constant motion, so as to drive the water onwards, and it is this constant cihary action in the walls of the branchial sac that gives rise to the all-important current of water stream- ing through the body. In addition to the stigmata there are generally one or two mu
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
