Zoology : an elementary text-book / [Sir A.E. Shipley].

  • Arthur Shipley
Date:
1904
Catalogue details

Licence: In copyright

Credit: Zoology : an elementary text-book / [Sir A.E. Shipley]. Source: Wellcome Collection.

    115/702 page 89
    CHAPTER YL Phylum Annelida. Segmentation. The name Annelida (Lat. annulus, a ring) means ringed, and refers to the fact that the bodies of the creatures grouped under this name are built up of a series of parts more or less resembling each other placed one behind another. This division of the body into more or less similar parts is called segmentation; each part is called a segment (or somite), and the animal is said to be segmented. Like the symmetry, the segmentation may be merely external or may affect both the exterior and a greater or less number of the internal organs. Sometimes, however, as in the case of the longer half of an earthworm’s body, the segmentation affects all the organs, and the likeness of one segment to another is so great that it would be impossible to say what part of the body any given isolated segment was taken from. More often, however, one or another of the organs of the body differs in shape or size in successive segments, and this is the case with the internal organs of the first twenty segments of the earthworm’s body, so that if these segments were all separated it would not be very difficult to place them together in their natural order. If we take an earthworm and kill it by placing it in alcohol for a few minutes and examine it carefully, we shall see Earthworm. that the body is composed of some 150 rings, each features! °f corresponds with a segment. The rings are separated from one another by slight grooves. At each end of the body there is an opening, the mouth (2, Fig. 40) in front and the anus (3, Fig. 40) behind. Besides these, two slit-like pores with rather swollen lips, situated on the under sur- face of the fifteenth segment (5, Fig. 40), may be seen. These are the pores through which spermatozoa are discharged, and are consequently known as the male genital openings. The
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