Product Code Database
This historic book may have numerous typos and missing text
Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated.1917 Excerpt: ... the germinal epithelium of the parthenitae arose from a common type of germ cell. In other words, the germ cells of cercariae and parthenitae are homologous. Stages in the phylogenetic development of the germinal epithelium may be outlined thus: 1. Germinal epithelium non-localized. Example, Cercaria diaphana (Fig. 79), C. micropharynx (Fig. 94). Sporocyst cycle. 2. Germinal epithelium localized; no mouth or suctorial apparatus. Example, C. glandulosa (Fig. 67). Sporocyst cycle. 3. Germinal epithelium localized; suctorial disc or attachment organ opposite germinal cell mass. Example, C. dendritica (Fig. 87), C. racemosa (Fig. 105). Sporcyst cycle. 4. Germinal epithelium localized opposite a true oral aperture, with pharynx and gut present. Example, C. flabelliformis (Fig. 43). Redia cycle. 5. Germinal epithelium localized and specialized into two sorts of conjugating germ cells, male and female isogametes. (Theoretical.) 6. Male germ cells proliferated in numbers from the mass of germinal tissue at the posterior end of the body; female germ cells more highly differentiated. Example, C. gracillima (Fig. 149). Cercaria stage of hermaphroditic cycle. 7. Germinal cells massed into a small number of specialized glands, called testes and ovaries. Example, C. pellucida (Fig. 18). Cercaria stage of hermaphroditic cycle. CERCARIA (GENERAL) The cercaria is the offspring of the parthenita. It is a highly specialized individual, homologous to the immature redia or the sporocyst. Its specialization has been accounted for by Ssinitzin (1910:38-56) because of 1) a considerable period of free-swimming life, during which it acquired a tail, and 2) a change to parasitism in the vertebrate, which was at first facultative, but later became obligatory. There are two types of mo...
|
