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girdle or clitellus, which is formed of a thick glandular layer, is 

 almost always present. 



The embryonic development of the Oligochceta presents many 

 relations to that of the Hirudinea. . The unequal segmentation, which 

 is very much alike in the two groups, and the similarity in the 

 method of origin of the mesoderm, from two large cells near the 

 blastopore at the posterior end of the embryo, point to a close relation- 

 ship between these two groups of Annelids. 



A few Oligochceta, as for example Chcetogaster, are parasitic on 

 aquatic animals ; the rest of them live, some free in the earth, some 

 in fresh water, and some in the sea. 



Sub-order 1. Terricolse. Oligochseta which live principally in 

 the earth. They have segnieiital organs in the genital segments. 



Fam. Lumbricidae. Large earthworms with compact skin and red blood. 

 Without eyes. Tufts of vessels surround the segmental organs. Their activity 

 in boring into the earth is of the greatest importance, loosening and exposing 

 the soil to the action of the weather. Lumbricu* L., Earthworm. Prasstomium 

 distinct from the mouth segment. The clitellus includes a series of segments, 

 and is situated nearly at the end of the anterior quarter of the body far behind 

 tlie genital openings. Setae elongated, hook-shaped, arranged in four groups in 

 each segment, each group containing two setaa. The earthworm lays its eggs 

 in capsules, into each of which several small ova, with sperm from the recep- 

 tacula seminis, are emptied ; as a rule, however, only one or but a few embryos 

 are developed. The developing embryo takes up with its large ciliated mouth 

 not only the common mass of albumen, but also the other eggs. Z. agricola 

 H<yR.-m.=*terrestri$ Lin., L. faetidus Sav., L. americanus E. Perr. Criodrilus 

 lacuum Hoffm. 



Sub-order 2. Limicolse. Oligochseta which live principally in 

 water. Without segmental organs in the genital segments. 



Fam. Phreoryctidse. Long filiform worms, with thick skin and two rows of 

 slightly curved setas on each side. Plireoryctes Menkeanus Hoffm. Found in 

 deep springs and wells ; they seem to feed on the roots of plants. 



Fam. Tubificidse. Aquatic worms, provided with four rows of simple or 

 divided, hooked setre. Hair-like -setae may also be present. The receptacula 

 are in the 9th, 10th, or llth segment. They live in mud tubes, from which 

 they protrude the posterior end of the body. Tubifex rivulorum Lam. The 

 heart is in the 7th, the receptacula in the 9tla segment. T. Bonncti Clap. 

 (Scenuris varicgata Hoffm.) The heart in the 8th, receptacula in the 10th 

 segment ; both species live in fresh water. lAmnodrilus Hoffmcisteri Clap., 

 L. D' Udekemianns Clap Is distinguished from Tubifex by the absence of 

 hair-like setae in the upper row of setae. Lumbriculus variegatus 0. Fr. Mull. 

 Every segment is provided with a contractile vascular loop and saccular 

 contractile appendages of the dorsal vessel. 



Fam. Naide*. Small Limieolcs with delicate thin skin and clear, almost 

 colourless, blood. The praestomium is often elongated like a proboscis and 



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