. Elements of comparative anatomy. Anatomy, Comparative. 202 COMPARATIVE ANATOMY. skeleton is merely represented by separate and microscopic deposits, these are generally definite in shape, and characteristic of genera and species. The calcareous skeleton of the larva forms an organ of support, which is generally made up by a framework of delicately attached, and often perforated rods. They are ordinarily found in the larval Echino'ida and Ophiurida; there are also calcareous bodies in the larvae of the Holothuroi'da. The presence of a calcareous skeleton in the larvas is clearly an instance o
. Elements of comparative anatomy. Anatomy, Comparative. 202 COMPARATIVE ANATOMY. skeleton is merely represented by separate and microscopic deposits, these are generally definite in shape, and characteristic of genera and species. The calcareous skeleton of the larva forms an organ of support, which is generally made up by a framework of delicately attached, and often perforated rods. They are ordinarily found in the larval Echino'ida and Ophiurida; there are also calcareous bodies in the larvae of the Holothuroi'da. The presence of a calcareous skeleton in the larvas is clearly an instance of an arrangement which is common to the group ; but it must not be forgotten that this larval skeleton corresponds to the form of the larva, and not to that of the adult Echinoderm; none of it passes permanently into the adult form. There is, in fact, a repeated change of the calcareous skeleton in the Holothuroi'da. § 164. As regards the special characters of the dermal skeleton, the presence of pieces, movably connected with one another, on the ambulacra! surface of the arms, is characteristic of the Aste- roi'da. Transversely - placed pairs of calcareous pieces, which gradually diminish in size, are found from the mouth as far as the tip of the arm (Fig. 100, A w); they form the floor of a groove—the tentacular groove. The sepa- rate pieces form a jointed series by their articular at- tachments, and the suckers pass out between the solid joints (p). These calcareous pieces are consequently known as ambulacral plates. But as special soft parts (ambu- lacral canal and nerves) are also embedded in this groove, the jointed segments do not appear to be purely dermo- skeletal parts. The ambu- lacral groove is covered by the integument, which is con- tinued laterally on to the am- bulacral plates. It consists largely of a layer of long cylindrical cells, covered by a cuticle. At the side it passes into a layer of cells, which is placed much deeper. At the lateral edges of the gr
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectanatomycomparative
