. A text-book of invertebrate morphology. Invertebrates. 560 INVEItTEBBATE MORPHOLOGY. terior extremity, forming a postoral ciliated band, the anus lying without the area enclosed by it. In front of the mouth is a trilobed region also surrounded by a band of cilia, the adoral band. In young embryos the adoral and postoral bands are united at the apex, separation only super- vening later. In later stages two additional arms are developed at the sides of the apical lobe, which becomes like the new arms destitute of cilia, and tipped with a group of wartlike elevations. This form of the larva is
. A text-book of invertebrate morphology. Invertebrates. 560 INVEItTEBBATE MORPHOLOGY. terior extremity, forming a postoral ciliated band, the anus lying without the area enclosed by it. In front of the mouth is a trilobed region also surrounded by a band of cilia, the adoral band. In young embryos the adoral and postoral bands are united at the apex, separation only super- vening later. In later stages two additional arms are developed at the sides of the apical lobe, which becomes like the new arms destitute of cilia, and tipped with a group of wartlike elevations. This form of the larva is known as the BracMolaria. A peculiar process, amounting almost to a metamorphosis, occurs during the transformation of the larva into the Starfish. Calcareous plates of the aboral system make their appearance on the dorsal surface of the stomach. Fig. 256.—^BiPiNNABiA of Asteraeantliion (after agassiz). an = anus. liy = bydrocoel. m = mouth. near the posterior end of the body, and oral plates on the ventral surface of the same organ. These two systems, at first rather widely separated, gradually approach each other, and at the same time the internal organs assume the adult form. Finally the two series of plates unite, enclosing between them the hydrocoel, a portion of the digestive tract and of the cce- lom. The original mouth and anus are obliterated, and indeed the anter- ior half of the larva takes no part in the formation of the adult animal, but is gradually absorbed. A highly-developed faculty for regeneration occurs in the Asteroidea, the disk being able to regenerate lost arms ; and indeed an arm, with which a small fragment of the disk is in connection, has the power of regenerat- ing all the missing parts. Specimens of the common Starfish Astenas are in consequence frequently found with one or more of the arms bifid at the tip, or even with an abnormal number of Please note that these images are extracted from scanned page images that may have been digitally
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Keywords: ., bookcentury1800, bookdecade1890, bookpublishernewyo, bookyear1894
