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Show 44 ,ON CLASSIFICArl'ION. The larvre of Asteridea and Holoth~widea are devoid of any continuous skeleton, but those of Ophiu1'"idea and Ech1:nidea possess a very remarkable bilatera~ly sym1netrical, continuous, calcareous skeleton, which extends Into, and supports the processes of the body (Fig. 20). A distinctly defined alirnentary canal early 1nakes its ap-pearance in these Echinoderm lm~vro. It is divided into a w~llmarked oral and oosophagea] portion, a globular st01n ach, and a short intestine tern1inating in an anal aperture (Figs.17 and 18). All the parts of the alimentary canal arc disposed in a longitudinal and vertical plane, dividing the larval body into two symmetrical halves; but the oosophageal and intestinal portions are so disposed as to make an angle, open towards the ventral side, with one another. No nervous, or other organs, besides those indicated, have as yet been discovered in these lar vrc. Fig-. 18. Fig. 18.-A youug Asterid larva (after l\Tull er).-A. Vent1 al. B. Lateral views of the larva. C. Commencin o- rudiment of the starfi sh. a. Mouth. b. msopltagus. c. Stomach. c'. 1ntestlne. o. Anus. x. Anterior, and y, prin cipal ci liated band. h. Crecal diverticulum, forming the rudiment of the ambulac1al Yase ular system, and opening externally by the pore, g. ll. Perisoma of the starfi sh. Fig. 19. Fig. 19.-Development of a B, C. Later stages. TJIF. ECHINODERMATA. 45 A Fig. 20.-Development of an EcMnid larva (after Mi:i.ller).- A. earli est, and B, later condition of larva. C. The Echinid imago developed within and nearly obliterating the l a rve~. After swimming about in this condition for a while, tho larva begins to show the first signs of those changes by which it is converted into the adult Echinoderm. A.. n involution of the integu1nent takes place upon one side of the dorsal region of the body, so as to give rise to a cmcal tube, which gr~dually elongates inwards, and eventually reaches a mass of formative matter, or blastema, aggregated upon one side of the stomach. Within this, the end of the tube becomes converted into a circular vessel, from which trunks pass off, radially, through the enlarging blastema. The latter, gradually expanding, gi'Ves rise in the Echinidea, the Asteridea, the Ophiuridea, and the Crinoidea, to the body-'Nall of tho adult; the larval body and skeleton (when the latter exists), with more or less of the primitive intestine, being either cast off as a whole, or disappearing, or becoming incorporated with the secondary devolopn1ent, while a new mouth is developed in the centre of the ring formed by the circular vessel. The vessels which radiate from the latter give off diverticula to communicate with the cavities of numerous proeesses of the body-the so ·cal1ed feet-which are the chief locomotive organs of the adult. The radiating and circular vessels, with all their appendages, constitute what is known as the "ambulacral system;" and, in Asterids and Echinids, this remarkable system of vessels ren1ains in communication with the exterior of the body by canals, eonnected with perforated portions of the external skeleton-the so-called ". madreporic canals" and "tubercles." In Ophiurids the persistence of any such communication of the an1bulacral system |
