. A manual of zoology. Zoology. 294 ECHIXODERMA Where there is no brood pouch the young escape from the egg as larvx (fia;. 2S2, /) which swim at the surface, and are distinguishable from the adults by their soft consistency, transparency, and bilateral symmetry. By the deyelopment of lobe-like processes and slender arms supported by calcareous rods the larys assume the most different and bi- zarre shapes [plutci of echinoids and ophiuroids {]'!). hracliiolaria (VII) and hipinnaria {VI) of asteroids, auricular id {III) of holothurians), all. Fig. 2S2.—Echinoderm larvcr (after J. ^liillerV a. a
. A manual of zoology. Zoology. 294 ECHIXODERMA Where there is no brood pouch the young escape from the egg as larvx (fia;. 2S2, /) which swim at the surface, and are distinguishable from the adults by their soft consistency, transparency, and bilateral symmetry. By the deyelopment of lobe-like processes and slender arms supported by calcareous rods the larys assume the most different and bi- zarre shapes [plutci of echinoids and ophiuroids {]'!). hracliiolaria (VII) and hipinnaria {VI) of asteroids, auricular id {III) of holothurians), all. Fig. 2S2.—Echinoderm larvcr (after J. ^liillerV a. anus; ";, mouth; the black line, the course of the ciliated bands. /. form common to all; //. ///. de\'elopmental stages of auricularia (Holothurian); 71'. T, stages of the -\stcroid bipinnaria; TV, pluteus of a spatangoid: 1'//, larva (brachiolaria) of Asterias (,orig.). ??;, mouth; v, vent. of which can be referred back to a common t^-pe with tri-regional alimen- tary tract and a ciliated band around the mouth, strikingly resembling tornaria, the larva of Balanoglosstis. The dift'erent appearances of the larvtr are due to the drawing out of the ciliated band into lobes and arms, and also to its liecoming broken into parts which unite themselyes into complete rings {]'). The metamorphosis of the bilateral larva into the radial adult is very compli- cated. It begins early with the formatioa of outgrowths from the archenteron (fig. 2S3), which become separated and form the anlagen of the ccelom and ambulacral system. It is dilTicuU to give a short summary of the development, partly from the differences in the separate groups, partly from the contradictions of authors. The following seems to be the most common, A vasopcriloiical di\erticulum (fig. 2S3, /;<-) arises from the bottom of the archenteron; this soon divides into right and left vesicles, the left acquiring a connection with the terior (madreporic opening). Each vesicle separates into anterior (/;) and post
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
