. The causes and course of organic evolution; . or Balanoglossus series, the Urochordata or ascidianseries, and the Cephalochordata or lancelet series. The firstof these seems for many reasons to be a greatly modified marinederivative from marine nemertean ancestry, in which thelarva has at times become distinctly pelagic, and the adultanimal has undergone highly divergent modification froman advanced and specialized nemertean ancestor. In linewith such a view, we would regard the proboscis as the j)er-manently unfliped or evaginated proboscis of a nemertean,into the interior of which the neme
. The causes and course of organic evolution; . or Balanoglossus series, the Urochordata or ascidianseries, and the Cephalochordata or lancelet series. The firstof these seems for many reasons to be a greatly modified marinederivative from marine nemertean ancestry, in which thelarva has at times become distinctly pelagic, and the adultanimal has undergone highly divergent modification froman advanced and specialized nemertean ancestor. In linewith such a view, we would regard the proboscis as the j)er-manently unfliped or evaginated proboscis of a nemertean,into the interior of which the nemertean proboscis-sheathhas been pushed forward as a condensed and collapsed noto-chord, and is surrounded internally by the masses of musclethat in earlier history gave it its flexible character. The basal 540 Causes and Course of Organic Evolution part of the proboscis attachment, and the entire area roundthe proboscis-sheath, have here become the collar. The an-terior expansion in front of the notochord and inside the pro- d^- ^pg-, -ug. tc. Fig. 26—Comparative diagrammatic views of cephalic region of, a nemer-tean, h nemerto-hemichord type, c protohemichord type, d Balanoglossus. , i. p. everting inner glandular proboscis part in a, b, that becomes p. proboscis in c-d, o. p. outer or muscular part of proboscis, v. p. g. ventralproboscis glands, p. s. proboscis sheath becoming notochord nc in d, d. g, v. gdorsal and ventral brain masses connected by transverse commissure tr. c, , V. n. dorsal and ventral nerves, p. c. proboscis cavity, c. c. collar cavities,t. c. trunk cavities, a. np., p. np. anterior and posterior neuropores. boscis known as the proboscis coelom or the anterior bodycavity would then represent a part of the nemertean body-cavity, that by joint extrusion of the proboscis and forward Phylogeny of Animals 541 growth of the notochord in the cephahc region, has becomeabstricted off between the outer and inner muscular pa
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