. A manual of zoology. Zoology. u 508 CEOUDATA. Order II. Tethyoidea (Ascidiaeformes). With the exceiition of the pelagic Pyrosomid^ all of the ascidi- ans are attached to rocks, etc., in the sea. The greater necessity for protection caused by this sedentary life has resulted in a great development of tlie cellulose tunic, which, enveloping the internal organs, gives these animals a swollen, somewhat shapeless appear- ance. Two openings, mouth and atrial opjening, lead into the interior, and the water which issues from these, when the animals are taken from the ocean, has given them the common
. A manual of zoology. Zoology. u 508 CEOUDATA. Order II. Tethyoidea (Ascidiaeformes). With the exceiition of the pelagic Pyrosomid^ all of the ascidi- ans are attached to rocks, etc., in the sea. The greater necessity for protection caused by this sedentary life has resulted in a great development of tlie cellulose tunic, which, enveloping the internal organs, gives these animals a swollen, somewhat shapeless appear- ance. Two openings, mouth and atrial opjening, lead into the interior, and the water which issues from these, when the animals are taken from the ocean, has given them the common name 6f ' sea-scpiirts.^ On removing the tunic, which is but slightly attached to the other parts except at mouth and atrial opening, a muscular sac is seen (fig. 515), the fibres running circularly and longitudinally. In- side this sac are the viscera, the pharyngeal region by far the most conspicuous. The mouth leads to a short tube with tentacles {t), and then to the p)harynx, a wide sac which ti I lies in a large cavity, the 2^eribrancliial chamber, the walls of the pharynx and I \3 the enclosing sp)ace uniting on the ventral I (,^_^ side (fig. 543). The pharyngeal walls are r—^ perforated like a net by small ciliated I gill slits, arranged in longitudinal and " transverse rows (fig. 540), throiigh which the water received from the mouth jDasses into the peribranchial chamber and thence i-j. 546.—Ciona intestinnlis, a ^ . ., , , bitof the waiiot the KiUsac to tne atrium, and so out to the external enlarged to show the giU , , slits. world. While the respiratory water thus passes out in a nearly direct course, the food particles which it contains pass into the digestive tract. By means of a ciliated tract (peripharyngeal band) just inside of the tentacles and surrounded by mucus secreted by the endostyle (or hypobranchial groove), the food is carried back to the oesophagus (oe) at the base of the gill chamber, and thence to the stomach (usually ])rovided with liver
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectzoology, bookyear1902
