Page:The Zoologist, 4th series, vol 1 (1897).djvu/180

152 Yet in a sense the dorsal pores play their part in the excretory process, since the fluid contained in the cœlom or body-cavity, as well as certain other substances which in some species of Earthworm are coloured, can be caused to exude through them. Sometimes the exudation is in drops, but some foreign species are able to squirt it to a distance of a foot, much as Peripatus does. In these cases the process is perhaps protective.

In a memorable article on the Earthworm, published some years ago, Prof. Ray Lankester says: "In the cuticle of the Earthworm a system of very minute canals exists, ... which might either be described in connection with the respiratory mechanism, or here, if we regard these ducts as excretory pores ... It is undoubtedly through these minute canals, which exist throughout the integument of the Earthworm, that water passes to the perivisceral cavity, and a dense fluid passes out." Ude tried a series of experiments to ascertain whether or not water was admitted through these pores, but he failed to satisfy himself that such was the case, though I have many times observed the denser fluid of which Prof. Lankester speaks issuing from them.

It is to Prof. Busk that we are indebted, through Prof. Lankester, for one of the best accounts of these apertures in English. In a remarkable paper on the Earthworm, published by the latter in 1865, we have an illustration of the integument of a Worm with all the various pores found on the dorsal surface carefully represented. "One of these orifices, situated in the median dorsal line of the segment, appears always to be larger than the others, and penetrates directly to the perivisceral cavity. That these openings form a very ready and frequent means of escape to the colourless fluid may be ascertained by handling a large Earthworm, when some considerable quantity is nearly invariably found to escape from its dorsal surface." Nor is this all. Prof. Busk says that the fluid expressed from these pores was of a dirty greyish colour, thin and opaque. Examined under the microscope, it contained numerous spherical particles and pyriform granular bodies, besides irregular organic particles. This coloured fluid differs with the species of Worm examined. In some, as the Brandling and Turgid Worm, it is