. Elementary text-book of zoology. Zoology. CYCLOSTOM I. 155 lips and often by filamentous processes, is circular in shape, though the lips can be applied together so as to form a median longitudinal slit. It leads into a funnel-shaped buccal cavity, which is without jaws and is armed on the soft palate as well as on the floor with horny teeth (fig. 001). At the bottom of the funnel is the tongue, which, moving up and down like a piston, enables the animal to attach itself by its mouth as by a sucker. The pharynx, which follows the mouth, communicates with the branchial sacs either directly or
. Elementary text-book of zoology. Zoology. CYCLOSTOM I. 155 lips and often by filamentous processes, is circular in shape, though the lips can be applied together so as to form a median longitudinal slit. It leads into a funnel-shaped buccal cavity, which is without jaws and is armed on the soft palate as well as on the floor with horny teeth (fig. 001). At the bottom of the funnel is the tongue, which, moving up and down like a piston, enables the animal to attach itself by its mouth as by a sucker. The pharynx, which follows the mouth, communicates with the branchial sacs either directly or by a special passage (Petromyzon). The intestinal canal passes straight to the rectum and is divided into stomach and intestine by a narrow region, the walls of which project so as to form a sort of valve. The liver is always well developed, but there is no swimming bladder. The gills (fig. 592) lie at the sides of the oesophagus in six or seven pairs of branchial sacs. These open on either side by external branchial passages into the same number of separate respiratory apertures. In Myxine on the other hand there is on each side, almost on the ventral surface, only one opening, into which all the external branchial passages of the same side open. On the other side the t-acs communicate with the oesophagus, but, except in A unitu- ccetes, never directly by simple openings but by internal branchial passages or, as in Petromyzon, by a common passage lying beneath the (esophagus into which passage all the other branchial passages open. The water flows in from the exterior through the external branchial openings or in Myxine through the nasal passage, and is driven by the contraction of the constrictor muscles of the branchial sacs either out by the same way (Petromyzon}, or into the oesophagus, and from this to the exterior through a special unpaired canal on the left side. The heart lies beneath and behind the branchial skeleton. Some of the vascular trunks pulsate, , the po
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
