An elementary text-book of biology, comprising vegetable and animal morphology and physiology / by J.R. Ainsworth Davis.

  • James Richard Ainsworth-Davis
Date:
1893
Catalogue details

Licence: Public Domain Mark

Credit: An elementary text-book of biology, comprising vegetable and animal morphology and physiology / by J.R. Ainsworth Davis. Source: Wellcome Collection.

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    far from tlie hind end of the body. This aperture is termed anus in the female, but cloacal opening in the male, because it is common to the digestive and reproductive organs. The female possesses a rounded genital aperture, situated in the mid-ventral line about one-third of the way back. In both sexes there is a minute excretory pore, placed mid-ventrally not far from the front end. 2. The body-wall (Fig. 15) consists of a firm, chitinous several- layered cuticle, covering an epidermis, internal to which is a layer of longitudinally directed muscle-cells. In a young specimen the cuticle is thin and the underlying epidermis a distinctly cellular layer, but in the adult the formation of a thick complex cuticle (c) seems to have caused degeneration in the epidermis {ep'), which is then a thin granular stratum through which numerous nuclei are scattered, and which is tra- versed by gelatinous fibres. It is, however, much thickened mid- dorsally, mid-ventrally, and, especially, laterally, to form the dorsal {dd), ventral (vd), and lateral lines (Id) to which reference has already been made. Each of these is lined by a layer of cuticle which is folded in so as to nearly divide the line into two. The muscle-layer is divided into four parts by the epidermal lines, and is made up of flat muscle-cells. Each of these is about 1- of an inch long, and consists of a transversely striated contractile fibre, internal to which is a nucleated protoplasmic part produced into a number of threads. The cell is directed longitudinally, with its fibre closely attached to the epidermis, its protoplasm projecting into the body-cavity, and its flat sides facing adjacent muscle-cells. The thread-like internal processes run to the dorsal line in the upper half of the body, and in the lower half of the body to the ventral line. 3. The Digestive Organs (Fig. 15) consist of a narrow tube which runs straight from mouth to anus or cloacal aperture. This alimentary canal or gut is divided into three well-marked regions (1) the fore-gut (stomodseum), (2) mid-gut (mesenteron), and (3) hind-gut (proctodseum). The essential difference between these is their mode of origin. In the course of development (1) and (3) are formed as infoldings or in-pushings from the exterior, and are therefore lined by ectoderm, while (2) is lined by endoderm. The fore-gut consists of a thick-walled gidlet (oesophagus) into
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