. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. INTERNAL ANATOMY. 79 certain resemblances to that of Amphioxus, in that it occurs in the form of distinct segmental tubules, or nephridia, each possessing a funnel-shaped opening into the body-cavity, and an opening to the exterior at the sur- face of the body. It was, in fact, the recognition, some twenty years ago, by Semper and Balfour, of the resemblance between the arrangement of the nephri- dia of the Annelids and the primary segmental ori- gin of the kidney of the Craniota that was chiefly instrumenta

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The Book Worm / Alamy Stock Photo
Image ID: RE09MP
. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. INTERNAL ANATOMY. 79 certain resemblances to that of Amphioxus, in that it occurs in the form of distinct segmental tubules, or nephridia, each possessing a funnel-shaped opening into the body-cavity, and an opening to the exterior at the sur- face of the body. It was, in fact, the recognition, some twenty years ago, by Semper and Balfour, of the resemblance between the arrangement of the nephri- dia of the Annelids and the primary segmental ori- gin of the kidney of the Craniota that was chiefly instrumental in placing the Annelid-theory of Verte- brate descent on a tempo- rarily firm basis. A dissection of the an- terior portion of the body of an earthworm, exposing ^.^ g^,.Anterior portion of earth- the nephridial tubules, is wo™ dissected open from above to show the nephridia and nervous system. (From shown in Fig. 39. A pair w. T. Sedgwick and E. B. WILSON'S of such convoluted tubules G.««a/ sw.-i'j..) pr, Prostomium (prseoral lobe), e.g. occurs in each segment, or Cerebral ganglion, which has receded from the prostomium from the ectoderm of ring, of the body, com- which it arose, com. Circumcesophageal mencing from the third, commissure surrounding the buecal tube t> (latter not represented). v.n.c. Ventral Physiologically, of course, nerve-cord. n. Segmental nerves. nph. Nephridia. sp. Dissepiments. they are directly com- parable to the renal tubules of the Chordata, and in their general features, allowing for the absence of a common duct, the similarity in the two cases is striking enough. But when this undoubted similarity is used as an argument for deriving the Vertebrate excretory system directly from that of the Annelids, we tread on very uncertain ground.. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the ori

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