. Zoology : for students and general readers . Zoology. 226 ZOOLOGY. The student, in familiarizing himself with the structure and mode of growth of the leech, the common earth-worm. Fig. 148.—Transverse section of a worm, of AmpWoxtis, and a higher -vertebrate contrasted, o, skin ; 6, dermal connective layer; c, muscles; d, segmental organ ; h, arterial, and i, venous blood-vessel; g^ intestine ; I, notochord.—After Haeckel. and the Nereis, will obtain a good idea of the essential char- acteristics of the entire class. Order 1. Hirudinea.—In the leech (Fig. 149), Hirudo medicinalis Linn., the
. Zoology : for students and general readers . Zoology. 226 ZOOLOGY. The student, in familiarizing himself with the structure and mode of growth of the leech, the common earth-worm. Fig. 148.—Transverse section of a worm, of AmpWoxtis, and a higher -vertebrate contrasted, o, skin ; 6, dermal connective layer; c, muscles; d, segmental organ ; h, arterial, and i, venous blood-vessel; g^ intestine ; I, notochord.—After Haeckel. and the Nereis, will obtain a good idea of the essential char- acteristics of the entire class. Order 1. Hirudinea.—In the leech (Fig. 149), Hirudo medicinalis Linn., the type of the first and lower order, the body is somewhat flattened and divided into numerous short, indistinctly marked segments, not bearing any bristles or appendages. The head is small, with no appendages, bear- ing five pairs of simple eyes, while each end of the body ter- minates in a sucker. The mouth is armed internally with three pharyngeal teeth arranged in a triradial manner, so that the wound made in the flesh of persons to whom the leech is applied consists of three short, deep gashes radiating from a common centre. The stomach (Fig. 150) is large, with large lateral diverticula or lobes, while the intestine is small. The nervous system consists of a "brain" and ven- tral ganglionated cord. The vascular system is complicated, consisting of a median • dorsal and a ventral vessel, and two lateral vessels ; all these anastomose or interbranch, and the blood which courses through them is red, but is said to contain no corpuscles. The segmental organs, so characteristic of the Annulata, are well develofied in the leech, consisting of about seventeen pairs of tubes opening at one end at regular intervals on the under side of the body, and ending in a non-ciliated coil (Fig. 149, r) in the leech, or in the smaller'fish-leech, Glep- sine, open into the venous sinus by ciliated, open Please note that these images are extracted from scanned page imag
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectzoology, bookyear1879
