. A manual of elementary zoology . Zoology. ANNELIDA : THE EARTHWORM, NEREIS 233 is a flat, hollow, vertical process of the body-wall, standing out at the side of its segment and serving to pull the animal along in creeping, or to row it in swimming. It is cleft into two principal lobes, a dorsal notopodium and a ventral neuropodium. Each of these is again divided into smaller lobes and bears at its base a slender process known as a cirrus. A stout, deeply embedded seta or aciculum supports the notopodium and another the neuropodium. The front end of the body is modified to form a definite hea
. A manual of elementary zoology . Zoology. ANNELIDA : THE EARTHWORM, NEREIS 233 is a flat, hollow, vertical process of the body-wall, standing out at the side of its segment and serving to pull the animal along in creeping, or to row it in swimming. It is cleft into two principal lobes, a dorsal notopodium and a ventral neuropodium. Each of these is again divided into smaller lobes and bears at its base a slender process known as a cirrus. A stout, deeply embedded seta or aciculum supports the notopodium and another the neuropodium. The front end of the body is modified to form a definite head. This consists of the prostomium and the peristomium. On the prostomium are situated dorsally a pair of prostomial tentacles and two pairs of eyes, each of which flE*—^ is a pit lined by pig- mented cells and en- closing a gelatinous mass which serves as a lens. Ventrally the prostomium. bears a pair of stout palps. The peristomium carries on each side two pairs of long, slender tentacular cirri, and probably Corresponds tO tWO , Apical tuft of cilia; eye; m., opening of fiic^H wompnts A mouth; mes., mesoderm ; £r., preoral ring of luscu acgnicLita. -ri cilia; , stomodseum (the pouch of ectoderm bilaterally Symmetri- which forms the mouth and gullet). cal animal which leads an active life always has a head, and if the animal be segmented there is a tendency for the foremost seg- ments to enter into the composition of the head. This is known as cephalisation. Behind the last segment is a conical region without parapodia which bears a pair of slender anal cirri and the terminal anus. The alimentary canal of Nereis is simpler than that of the earthworm, but the pharynx can be caused to protrude by being turned inside out, and is lined with cuticle, thickened in places to form numerous small teeth and a pair of strong jaws with which the prey is seized. The sexes are separate. The reproductive organs are very simple,. FIG. 149.—The trochosphere of Nereis. —Modifi
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectzoology, bookyear1920
