Fig. 259. Diagrammatic representation of various forms of Asteroid Larvæ. A, B, C, Bipinnaria; D, Brachiolaria. (Copied from Müller.)
The black lines represent the ciliated bands; and the shading the space between the præ-oral and the post-oral bands.
m. mouth; an. anus.
As a rule the following arms can be distinguished ([fig. 259] C and D), on the hinder ring (Agassiz’ nomenclature) a median anal pair, a dorsal anal pair, and a ventral anal pair, a dorsal oral pair, and an unpaired anterior dorsal arm; on the præ-oral ring a ventral oral pair, and sometimes (Müller) an unpaired anterior ventral arm.
The three brachiolar arms arise as processes from the base of the unpaired dorsal arm, and the two ventral oral arms. The extent of the development of the arms varies with the species.
The changes by which the Bipinnaria or Brachiolaria becomes converted into the adult starfish are very much more complicated than those which take place in Holothurians. For an accurate knowledge of them we are largely indebted to Alex. Agassiz (No. [543]). The development of the starfish takes place entirely at the posterior end of the larva close to the stomach.
On the right and dorsal side of the stomach, and externally to the right peritoneal space, are formed five radially situated calcareous rods arranged in the form of a somewhat irregular pentagon. The surface on which they are deposited has a spiral form, and constitutes together with its calcareous rods, the abactinal or dorsal surface of the future starfish. Close to its dorsal, i.e. embryonic dorsal, edge lies the dorsal pore of the water-vascular system (madreporic canal), and close to its ventral edge the anus. On the left and ventral side of the stomach is placed the water-vascular rosette, the development of which was described on p. [549]. It is situated on the actinal or ventral surface of the future starfish, and is related to the left peritoneal vesicle.
Metschnikoff (No. [560]) and Agassiz (No. [543]) differ slightly as to the constitution of the water-vascular rosette. The former describes and figures it as a completely closed rosette, the latter states that ‘it does not form a completely closed curve but is always open, forming a sort of twisted crescent-shaped arc.’
The water-vascular rosette is provided with five lobes, corresponding to which are folds in the larval skin, and each lobe corresponds to one of the calcareous plates developed on the abactinal disc. The plane of the actinal surface at first meets that of the abactinal at an acute or nearly right angle. The two surfaces are separated by the whole width of the stomach. The general appearance of the larva from the ventral surface after the development of the water-vascular rosette (i) and abactinal disc (A) is shewn in [fig. 260].