Volume 1

A student's text-book of zoology / by Adam Sedgwick.

  • Sedgwick, Adam, 1854-1913.
Date:
1898-1909
Catalogue details

Licence: In copyright

Credit: A student's text-book of zoology / by Adam Sedgwick. Source: Wellcome Collection.

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    changing tlieir form, while others are amoehoid. They vary much in size, some not exceeding ten micromillimetres in length, while others may attain a length of sixteen mm. The protoplasm usually exhibits a differentiation into ectoplasm and endoplasm, and the endoplasm is often highly granular—a feature which becomes more marked with the age of the individual. Conjugation occurs in some grou]rs {Gregarinida, Drepanidiidia, • etc.), but has not been observed in all. Reproduction is effected by the division of the protoplasm into spores, which may be coated or naked. In most cases encystment r-oeWcv . precedes sporulation, but in some of them the spores are produced gi’adually during the ordinary life of the individual. In the Gregarinida and Coccidiidea the spore-protoplasm divides to form the young forms. In others the whole spore becomes the young^^'^*’ form. There is often a little residual protoplasm—generally non- nucleated—left over after the sporulation. The young which issue from the spores are either falciform or amoeboid. In^regarmida and Coccidiidea)the sporulation usually takes place after the cyst has left the host (exogeny), but in the other forms it is effected within the host (endogeny). A process which may or may not he analogous to the formation of polar bodies of the metazoan ovum (or speaking more generally, to the reduction- divisions of the proganietes) has been observed by Wolters in Gregariiies {Arch. Mic. Anat. Bd. 37) and by Labbe {loe. cit.) in Coccidia. The nucleus divides ; one half remains in the animal, while the other passes to the surface and disappears. This phenomenon precedes the fusion of the nuclei in the con- jugating gregarine, and sporulation in the Coccidia, In Gregarinida and Coccidiidea the cysts pass out with the faeces and enter another host in its food. In the other orders the method of transference from host to host is not certainly known, but probably in some cases the spores are not able to leave the host until its death, after which they may enter a new host in the food; while in other cases it is possible that the infection is transmitted by blood-sucking insects, or through the lungs in dust. It is possible that there may be—in the exogenous forms at least —some other mode of reproduction besides that of spore-formation. Very little is known on this head; but unless there is some other reproductive process, it is difficult to see how in exogenous forms, such as Coccidium oviforme of the rabbit, the enormous number of individuals which characterise acute coccidiosis is produced. It is also probable that in some cases, e.g., the forms which live in