Willis had previously written an account of these "foramina on the top of the back, adjoining to each ring, supplying the place of lungs." Now Willis published his work, 'De Anima Brutorum,' in 1672, so that for upwards of two hundred years the pores have been known to science, to go no further back. It is only in recent years, however, that they have been carefully noted, and the position of the first pore recorded for the different species of worm. It has been thought by some that the first dorsal pore was so uniformly placed in the various species of Earthworms that a specific character might be based thereon. This I am disposed to think is not borne out by facts.
Dr. Benham, one of our few English authorities on the subject, says: "In many Earthworms the cœlom is put into communication with the exterior by means of a series of dorsal pores, placed on the intersegmental grooves. In Lumbricus these pores occur in every somite after about segment eight; in Digaster and Perionyx they commence just behind somite four; in Plutellus behind somite six; in Pleurochæta and Typhæus the pores are present only behind the clitellum. They are present in Acanthodrilus, and in many Perichæetæ.'[1] In Allurus they begin behind segment three or four.
As will be inferred from the foregoing, a variety of ideas have prevailed respecting the use to which these apertures were devoted in worm economy. Willis says they supply the place of lungs, and if Derham's remarks apply to the dorsal pores, he regards them simply as the openings through which lubricants were poured. Lloyd Morgan is as cautious on the subject as he is inaccurate. He says: "Every segment of the body except the first has a dorsal pore opening into the anterior part of the ring in the mid-dorsal line, and two very minute pores, one on each side of the ventral line, which are the external orifices of the nephridia or segmental organs, whose function is excretory."
The dorsal pores are not found in the typical Earthworm on every segment save the first, and if they were, we are not favoured by the Professor with a vestige of an idea as to their use. He says: "There are no specially differentiated respiratory organs, respiration being apparently effected by the surface of the body," so that he does not regard the dorsal pores as lungs.
- ↑ Q.J. Mic. Sc. 1886, p. 247.
