. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. TYPES OF 281 a 225) of different form. These processes are arranged with a strict regard to bilateral symmetry, and their number, position, and size essentially determine the special shape of the body. An anterior and a posterior ventral region of the band of cilia can be distinguished from the lateral parts which form the dorsal portions; the latter curve round and pass into the former at the anterior and posterior ends of the body (fig. 225, b). The dorso-lateral parts may, howeve
. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. TYPES OF 281 a 225) of different form. These processes are arranged with a strict regard to bilateral symmetry, and their number, position, and size essentially determine the special shape of the body. An anterior and a posterior ventral region of the band of cilia can be distinguished from the lateral parts which form the dorsal portions; the latter curve round and pass into the former at the anterior and posterior ends of the body (fig. 225, b). The dorso-lateral parts may, however, unite anteriorly with one another without passing into the anterior ventral band; in this case the anterior continuations of the latter pass directly into one another so Us to form an independent praeoral ring, while the dorso-lateral and posterior ventral portions of the origin- ally continuous band form a longitudinally directed post-oral ring. This arrangement is characteristic of the larvae of the Asteridea (Bipinnaria, Brachiolaria). In all other forms a single longitudinal band of cilia only is pre- sent. In the larvae of Holo- thurians, the Auricularia (fig. 225), the processes re- main short and soft; they are found on the dorso- lateral edges and on the posterior dorso-ventral arch of the band of cilia; they also appear on the posterior ventral (umbrella) and the anterior ventral (oral shield) parts of the band. The processes have a similar disposition in Bipinnaria, where, however, they are often much longer, but are in this case also not provided with calcareous rods. The Brachiolaria are distinguished from the Bipinnaria by the possession of three anterior arms, which are placed between the anterior portions of the two rings of cilia, and serve as a fixing apparatus. The bilateral larvae of the Ophiurids and Sea- Urcldns, the so-called Pluteus forms, are distinguished by their large rod- shaped processes, which are supported bv a system of calcareous Pleas
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