. A textbook of invertebrate morphology [microform]. Invertebrates; Morphology (Animals); Invertébrés; Morphologie (Animaux). 162 IN VERTEBRA TE MORPHOLOO 7. IV. Class Xemertina. The three precediug classes show marked evidences of genetic affinity, the char;ieteri:stic differences of structure in the Trematodes and Ce«todf^s being due to the parasitic habits of these forms. The Nemerteans, on the othv . 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 b7^]ier grade than the othe
. A textbook of invertebrate morphology [microform]. Invertebrates; Morphology (Animals); Invertébrés; Morphologie (Animaux). 162 IN VERTEBRA TE MORPHOLOO 7. IV. Class Xemertina. The three precediug classes show marked evidences of genetic affinity, the char;ieteri:stic differences of structure in the Trematodes and Ce«todf^s being due to the parasitic habits of these forms. The Nemerteans, on the othv . 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 b7^]ier grade than the other Platyhelmiuths. They are for the most part 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. the digestive tract. This (Fi- 84 rf, is Ell-HAUDI (after Montgo- i - , ⢠, , ^ ^" * '' '^ MERv). ^" almost straight tube, except in the h = blood-vessels. genus J/alacoMelh, and is pushed out ou c. = cerebral ganglion, each side into sac-like pouches, which cfi'= ciliated funnel. â,.a ov,.; ,. i ⢠. ^ = intestine. ^'^ anauged m some cases with a oc = eyes. regularity of succession almost meta- ov = ovary. meric. It opens to the exterior at the F-= proboscis. anterior end of the body bv the mouth rm = retractor muscle. ^ short non-muscular oesophagus inter! veiling between the intestine proper and
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