. A manual of elementary zoology . Zoology. 176 MANUAL OF ELEMENTARY ZOOLOGY are set free by breaking away as the buds of Hydra are, and carry out sexual reproduction as inde- pendent individuals. These individuals differ widely from the polyps, being, indeed, so unlike them that their origin from the colony would never have been guessed unless it had been seen to take place. They are small jelly-fish or Each has the shape of an umbrella with a short, thick handle anda fringe oftentaclesaround the edge. The convex upper side is called the exum- brella, the concave lower side th
. A manual of elementary zoology . Zoology. 176 MANUAL OF ELEMENTARY ZOOLOGY are set free by breaking away as the buds of Hydra are, and carry out sexual reproduction as inde- pendent individuals. These individuals differ widely from the polyps, being, indeed, so unlike them that their origin from the colony would never have been guessed unless it had been seen to take place. They are small jelly-fish or Each has the shape of an umbrella with a short, thick handle anda fringe oftentaclesaround the edge. The convex upper side is called the exum- brella, the concave lower side the subumbrella, the handle the manu- brium. Around the edge of the umbrella a low ridge projects in- wards. This is the velum and re- presents a much larger structure in the same region of many other medusae. At the Fig. no.—A longitudinal section of a hydranth end of the manu- of Obelia, highly magnified. brium is the Ectoderm ; end., endoderm ; endoderm of the mouth which tentacles : hydrotheca ; oral cone ; , , , . ' , . structureless lamella. leads by a tubular gullet along the manubrium to a stomach in the middle of the body. From this four radial canals run outwards to a ring canal at the edge of the umbrella. The lining of all these internal spaces consists of endoderm, and the radial canals lie in a sheet of endoderm, known as the endoderm lamella. In fact we may regard the internal cavities of a medusa as corresponding to the enteron of a polyp in which the walls have come together over a large area, leaving certain. 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 original Borradaile, L. A. (Lancelot Alexander), 1872-1945. London : H. Frowde, Hodder & Stoughton
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectzoology, bookyear1920
