An introduction to the systematic zoology and morphology of vertebrate animals / by alexander Macalister.

  • MacAlister, Alexander, 1844-1919.
Date:
1878
Catalogue details

Licence: Public Domain Mark

Credit: An introduction to the systematic zoology and morphology of vertebrate animals / by alexander Macalister. Source: Wellcome Collection.

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    / Reproduction and Embryogeny of Amphioxus. 9 branch supplies the mouth. The rest of the nerves termi- nate either in the muscles or under the epithelium of the surface. Over the neural canal, in a longitudinal space formed by the splitting of the outer chordal sheath, is a band of tissue pierced by canals like the material of the hypodermis (5, e); over this, in the medio-dorsal fin, is a longitudinal row of spaces, quadrate in horizontal, pyriform in transverse section (5, i), filled with elastic endothelium-clad bodies. Similar spaces lie laterally and medio-ventrally, extending between the abdominal pore and the anus; the true fin is that which stretches behind the anus. Two blind lateral canals stretch in the side-folds from the side of the mouth-cavity to behind the abdominal pore (their existence has been denied, but the careful dissections of Rolph have shown their presence). There is a contrac- tile vessel or vena portce beneath the liver (4, c), whose blood enters the h}q>opharyngeal heart {e) with the rest of the systemic venous blood. From the simple cavity which is called the heart, laterally arched branchial arteries spring, each having a contractile swelling at its base, and passing in the wall of the pharynx between the gill slits, to end in branchial veins which terminate in a dorsal aorta. The two most anterior branchial arches [g] are large, like aortic arches. In the dorsal trunk (A-) the blood flows tailwards, in the heart mouthwards. The blood corpuscles are amoe- boid, nucleated, colourless. On each side, under the floor of the atriiun in the coelo- ma, are quadrate or bean-shaped masses, arising probably from the atrial epithelium ; these may contain either eggs or spermatozoa, and in both sexes they empty their contents into the atrium by dehiscence, and these escape through the gill slits, or more rarely through the abdominal pore.