. Elements of comparative anatomy. Anatomy, Comparative. ALIMENTARY CANAL OF ECHLNODERMA. 213 form several rows one above the other (Fig-. 102, d). In these therefore the dermal skeleton forms the organs for the comminution of the food. A short, wide oesophagus follows the mouth; this is continued into a wide mid-gut (stomach), which occupies the middle of the body. In the Ophiurida and many Asterida (Astropecten,. Fig. 107. Transverse section through the arm and disc of Solaster en dec a. The radial and the interradial portions are figured on opposite sides. o Mouth. v Stomachal cavity, c Rad
. Elements of comparative anatomy. Anatomy, Comparative. ALIMENTARY CANAL OF ECHLNODERMA. 213 form several rows one above the other (Fig-. 102, d). In these therefore the dermal skeleton forms the organs for the comminution of the food. A short, wide oesophagus follows the mouth; this is continued into a wide mid-gut (stomach), which occupies the middle of the body. In the Ophiurida and many Asterida (Astropecten,. Fig. 107. Transverse section through the arm and disc of Solaster en dec a. The radial and the interradial portions are figured on opposite sides. o Mouth. v Stomachal cavity, c Radial ceeca. g Genital gland, m Madreporic plate, s Stone- canal with its so-called heart, p Ambulacra! feet (after G. 0. Sars). Luidia) the stomach is always a blind sac, as it is also in Brisiuga. But in all Astero'i'da it is provided with diverticula or cascal saccular appendages, which are indicated in the Ophiurida by radial constrictions. The gastric casca of the Asterida extend in pairs into the arms; they spring from the stomach, and have the form of thin-walled tubes, closely beset with lateral appendages (Figs. 107, c; 108, h), which as a rule are united by pairs into one canal before they open into the stomach. This tract represents an unpaired por- tion of the enteron belonging to each antimere (arm) of the Astero'i'da, while the cascal tubes form a paired por- tion. In Astropecten auran- tiacus these tubes arise sepa- rately from the stomach. The unpaired portion in each arm has therefore disappeared in this form, and with it the primitive condition. In most of the Asterida the short hind-gut is continued from the stomach to the anus, which is placed on the dorsal surface. The enteric tube of the Crino'i'da (Comatula) is modified; it describes a spiral coil, and its narrower short terminal portion passes into a tubular and projecting anus, which is placed interradially near the mouth. This coiled arrangement, which is apparently very anomalous. Please note that these
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectanatomycomparative
