A phylogenetic classification of animals (for the use of students) . ir first ancestors (fig. 16)differed from the ancestors of the Enteropneusta in thatportion of the coelom (diverticula from the archenteron),which formed the water-vascular system being developed* See Balfour, Comp. EmbryoL, vol. i, p. 485. 41 radially, or as a series of pouches around the front part ofthe alimentary canal, so as to produce the first traces of themore or less complete radial symmetry which is such amarked feature in all Echinoderms. If the nervous systemwas still in its primitive difiused condition, it may re
A phylogenetic classification of animals (for the use of students) . ir first ancestors (fig. 16)differed from the ancestors of the Enteropneusta in thatportion of the coelom (diverticula from the archenteron),which formed the water-vascular system being developed* See Balfour, Comp. EmbryoL, vol. i, p. 485. 41 radially, or as a series of pouches around the front part ofthe alimentary canal, so as to produce the first traces of themore or less complete radial symmetry which is such amarked feature in all Echinoderms. If the nervous systemwas still in its primitive difiused condition, it may readilybe imagined that the formation of radially arranged regionsof the coelom, which were being evolved into vessels withtentacle-like projections to the exterior, would result in theconcentration of the nervous tissue along these lines; andif, as is probable, there was previously a nervous concentra-tion around the mouth, then the newly formed nerve bandswould naturally radiate outwards from the circum-oral ring(see fig. 16). The other characters which the common. Fig. 16. Hypothetical ancestor of the Echinodermata. m, mouth; a, anus;n, nerve cord; r, water-vascular diverticulum from archenteron. ancestor of the Echinodermata must have acquired are:—atendency towards the formation of the water-vascular caecain fives, and as a result the pentagonal symmetry of most ofthe systems of the body; the development of a large ccelomor body cavity; and the deposition of calcareous matter inthe deeper parts of the integument. The ancestral line then probably split into two series :—the one being continued into the progenitors of theAsteroids, the Ophiuroids, and the Crinoids, and the otherbeing evolved into the primitive Echinoids and Holothurians. In the latter series the primitive body-form was most nearlyretained, but all the systems became considerably differen-tiated. In the ancestral Holothuroidea the shape became elon-gated antero-posteriorly, and, as a result, the pentagonala
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1885
