. A text-book of invertebrate morphology. Invertebrates. 162 INVERTEBRATE MORPHOLOGY. IV. Class Nemertina. The three preceding classes show marked evidences of genetic affinity, the characteristic differences of structure in the Trematodes and Cestodes being due to the parasitic habits of these forms. The Nemerteans, on the other hand, though apparently tracing descent from a Turbellarian-like ancestor, show a marked advance in structure, and must be regarded as organisms of a considerably higher grade than the other Platyhelminths. They are for the most jDart marine, though a few forms inhabi
. A text-book of invertebrate morphology. Invertebrates. 162 INVERTEBRATE MORPHOLOGY. IV. Class Nemertina. The three preceding classes show marked evidences of genetic affinity, the characteristic differences of structure in the Trematodes and Cestodes being due to the parasitic habits of these forms. The Nemerteans, on the other hand, though apparently tracing descent from a Turbellarian-like ancestor, show a marked advance in structure, and must be regarded as organisms of a considerably higher grade than the other Platyhelminths. They are for the most jDart marine, though a few forms inhabit fresh water or even damp earth, and are usually elongated ribbon-like forms, reaching a length, in some cases, of several centimetres. The body is ex- ternally unsegmented, though a more or less marked metamerism of the internal organs, due to their repetition at definite intervals, may be present. The ectoderm of the body resembles that of the Tur- bellaria in being throughout ciliated, and rests upon a basement membrane, which in some cases contains cells. Within the membrane are a varying number of muscle layers, differentiations of the outermost portions of the mesodermal tissue, which in the form of a parenchyma occupies the interval between them and Fig. 84—AnteriorPoR-tlie digestive tract. This (Fig. 84, d) is TioN OF Nbmektban. an almost straight tube, except in the genus Malacohdella, and is pushed out on each side into sac-like pouches, which are arranged in some cases with a regularity of succession almost meta- meric. It opens to the exterior at the anterior end of the body by the mouth, a short non-muscular oesophagus inter- vening between the intestine proper and that opening; and at the other end of the body is a second communication with. ce = cerebral ganglion. eg = ciliated funnel. d = intestine. oc = eyes. 011 = ovary. pr = proboscis. rm = retractor muscle of Please note that these images are extracted from scanned page images that may have be
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