A second abnormal type of development is presented by the embryo of Pteraster miliaris, as described by Koren and Danielssen[222]. The larvæ to the number of eight to twenty develop in a peculiar pouch on the dorsal surface of the body. The early stages are not known, but in the later ones the whole body assumes a pentagonal appearance with a mouth at one edge of the disc. At a later stage the anus is formed on the dorsal side of an arm opposite the mouth. The stomach is surrounded by a water-vascular ring, from which the madreporic canal passes to the dorsal surface, but does not open. At a later stage the embryonic mouth and anus vanish, to be replaced by a permanent mouth and anus in the normal positions.
A third, and in some respects very curious, form is a worm like larva of Müller, which is without bands of cilia. The dorsal surface of the youngest larva is divided by transverse constrictions into five segments. On the under side of the first of these is a five-lobed disc, each lobe being provided with a pair of tube feet.
At a later period only three segments are visible on the dorsal surface, but the ventral surface has assumed a pentagonal aspect. The later stages are not known.
Fig. 261. Diagrammatic figures shewing the evolution of an Ophiuroid Pluteus from a simple Echinoderm larva. (Copied from Müller.) The calcareous skeleton is not represented.
m. mouth; an. anus; d. anterior arms; d´. lateral arms; e´. posterior arms; g´. anterolateral arms.
Ophiuroidea. The full-grown larva of the Ophiuroids is known as a Pluteus. It commences with the usual more or less spherical form; from this it passes to a form closely resembling that of Auricularia with a rounded dorsal surface, and a flattened ventral one. Soon however it becomes distinguished by the growth of a post-anal lobe and the absence of a præ-oral lobe ([fig. 261] B). The post-anal lobe forms the somewhat rounded apex of the body. In front of the mouth, and between the mouth and anus, arise the anal and oral ciliated ridges, which soon become continued into a single longitudinal ciliated ring. At the same time the body becomes prolonged into a series of processes along the ciliated band, which is continued to the extremity of each. The primitive ciliated ring never becomes broken up into two or more rings. A ciliated crown is usually developed at the extremity of the post-anal lobe. The arms are arranged in the form of a ring surrounding the mouth, and are all directed forwards.
The first arms to appear are two lateral ones, which usually remain the most conspicuous ([fig. 261] B and C, d´). Next arises a pair on the sides of the mouth, which may be called the mouth or anterior arms (C, d). A pair ventral to and behind the lateral arms is then formed, constituting the posterior arms (D, e´), and finally a pair between the lateral arms and the anterior, constituting the anterolateral arms (D, g´).
The concave area between the arms forms the greater part of the ventral surface of the body. Even before the appearance of any of the arms, and before the formation of the mouth, two calcareous rods are formed, which meet behind at the apex of the post-anal lobe, and are continued as a central support into each of the arms as they are successively formed. These rods are shewn at their full development in [fig. 262]. The important points which distinguish a Pluteus larva from the Auricularia or Bipinnaria are the following: