The most important contribution to the subject is undoubtedly that which was made a few years ago by Hermann Ude, in a paper which deals chiefly with the structure of the bodywall in Earthworms.[1] He points out that "the dorsal pore lies on the anterior edge of the somites in which it occurs, and appears on the intersegmental groove. It is absent in the most anterior somites, but the position of the first pore is constant for a given species." In the Common Earthworm it occurs between eight and nine, and in the Turgid Worm between ten and eleven. We should say between eight and nine and nine and ten respectively. Claparède formerly described the epidermis as being folded inwards at the dorsal pore, just as it is where the setæ are situated, but Ude shows that such is not the case. By stripping off the epidermis I have been able to detect the infolding of the cuticle around the setæ, but not around the dorsal pore, which, as Ude affirms, is a perforation through the epidermis and the muscular layers. The pore is wanting in most Freshwater Worms or Limicolæ. Beddard has dealt with the exceptions. In some worms, when the girdle is fully developed, the pores become closed through the growing up of the cuticle around the edge. This is not always the case, however, for the Mucous Worm has been noted by some to be an exception, while I have found that the dorsal pore on the clitellum or girdle of some species is quite as discernible after the organ has attained full development as before.
If a worm is opened laterally, and the internal organs removed so as to leave only the body-wall, it will be possible so to display this portion of the animal as to see the whole series of pores in regular succession. It will be easy then to observe that they are connected with each other by a kind of tube which runs right along the back of the worm. I am a little doubtful whether or not this is what Ude refers to when he says that the epithelium of the body cavity passes across the muscular layers, and meets the cuticle around the edge of the pore. The pore has a special set of muscle-bundles which form its sphincter muscle.
Ude does not think there is the slightest connection between the pores and the nephridia, which are excretory in their function.
- ↑ Zeit. f. Wiss. Zool. xlvi. pp. 85–142. Benham, Q. J. Mic. Sc. Aug. 1886, No. cv. pp. 102–4.
