A treatise on zoology . cirri, often with traces of polymercs like those of the primitive stem, often very long and branching again, and always with a lumen which contains an extension of the axial cord (Fig. CXVI.); this is adapted to a muddy bottom. The radical cirri arise from the vertical suture-lines of the stem, by the intercalation and outgrowth of small plates, and the extrusion of the axial cord (Fig. XVI. 3). X ,1 p -x • , ,^ • • 1 11 Thomson, X 20 (liam. In the course oi race-nistory the cirri gradually ap- coz, fascicular stereom pear higher and higher up the stem (Fig. XVI. 1), Sr
A treatise on zoology . cirri, often with traces of polymercs like those of the primitive stem, often very long and branching again, and always with a lumen which contains an extension of the axial cord (Fig. CXVI.); this is adapted to a muddy bottom. The radical cirri arise from the vertical suture-lines of the stem, by the intercalation and outgrowth of small plates, and the extrusion of the axial cord (Fig. XVI. 3). X ,1 p -x • , ,^ • • 1 11 Thomson, X 20 (liam. In the course oi race-nistory the cirri gradually ap- coz, fascicular stereom pear higher and higher up the stem (Fig. XVI. 1), : and at the same time become shorter, more mobile, and eventually arranged in whorls (Fig. XVI. 5). the axial cords of the stem swell out, forming a repetition of the chambered organ (Fig. XVI. 4). The columnals are rarely all of the same height; certain con-spicuously larger ones, including those that bear cirri, are termed Fm. XV. Distal end of sternof larval Antedon bi-fida, after Wyville- At these levels. Fig. XVI. Evolution of cirri. 1, part of stem of a Silurian crinoid with large, branching cirri ( E1354). 2, section across stem of a Carboniferous crinoid, showing branch from axialcanal to cirrus (Brit. i^Ius. E6708). 3, root of Barycriyms, with cirri originating betweenpentameres (niodilied from Waclismuth & Springer). 4, section across stem of IsocrimisWyvilh-Tkoinsoni at level of cirrus-whorl, the central portions disproportionately enlargedfor greater clearness. 5, part of stem of Isocrinus decorus, with cirri in whorls of five. nodals; those between them internodals (Fig. XVII. 1).Nodals are the first columnals to be formed during growth;internodals are subsequently intercalated between them, andagain fresh internodals between the first formed internodals, andso on. Fresh nodals are developed at the proximal end of thestem, so that in that region are more nodals, while distally aremore internodals (Fig. XVII. 2 and 3). In one type nodals areintr
Size: 2600px × 962px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, bookpublishe, booksubjectzoology
