. Elementary text-book of zoology. Zoology. poric plate into the body cavity, and is kept in active movement by the cilia which line the body cavity and the pevihreinal canals; in this way the surface of the internal organs is continually bathed by water. The leaf-like and pinnate ambulacral appendages (ambulacral branchice) of the irregular Sea-urchins are regarded as special organs of respiration, as also are the crecal tubes (dermal braiichia-), which project from the surface of the integument and communicate with o the body cavity in some regular Sea-urchins and in the Asteridea. These der
. Elementary text-book of zoology. Zoology. poric plate into the body cavity, and is kept in active movement by the cilia which line the body cavity and the pevihreinal canals; in this way the surface of the internal organs is continually bathed by water. The leaf-like and pinnate ambulacral appendages (ambulacral branchice) of the irregular Sea-urchins are regarded as special organs of respiration, as also are the crecal tubes (dermal braiichia-), which project from the surface of the integument and communicate with o the body cavity in some regular Sea-urchins and in the Asteridea. These dermal branchiae are dis- tributed in the Asteridea over the whole dorsal surface as simple tubes, and in the Echini they surround the mouth as five pairs of branched tubes. Lastly there are the so-called respiratory trees of Holothurians; these are two large tree-like branched tubes which open by a common stem into the cloaca. The water which is taken into these organs can be again ejected with great force (fig. 219). The nervous system (fig. 220) consists of five principal nerves running down the five rays. These nerves in the Asteridea lie imme- diately beneath the epidermis of the ambulacral groove, external to the radial blood vessel and water vascular trunk : they send off numerous fibres to the ambulacral feet, the muscles of the spines, pedicellarise, etc. These ectodermal bands may be looked upon as the central part of the nervous system ("ambulacral brains" of J. Miiller). Near the mouth they divide into two parts, which unite with corresponding branches from the other radial trunks to form a nervous ring containing ganglion cells. The tentacle-like arnbulacral feet which in the Asteridea and Ophiuridea are present in simple number at the end of the arms are supposed to have the value of tactile organs. The same significance has been attributed to the tentacles of the Holothurids and to the pencil-like tactile feet of the Spatangidw. Organs resembling eyes FIG.
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
