OCR Text |
Show 30 MR. F. E. BEDDARD ON EARTHWORMS [Jail. 5, The prostomium is large, being larger than the diameter of the first segment. The seta: are disposed precisely as in Microscolex modestus : that is to say, the setae are not in closely approximated pairs, and the distance between seta 1 and seta 2 is less than that between setae 3 and 4. The diagram given by Rosa of the setae of M. modestus would express, so far as I can make out, the relations of the setae in M. algeriensis exactly. There is nothing noteworthy in the form of the setae. As in other Earthworms, the four setae of each side of the body in each segment are connected by muscular strands which favours, it may be supposed, their simultaneous movement. This muscle in Microscolex is easily overlooked, owing to its great thinness ; it is not more than two fibres thick. The clitellum is complete (forming, that is, a ring) and occupies segments xiv.-xvii. with a part of xiii. Its structure is like that of other Earthworms. I could find no dorsal pores. The alimentary tract is peculiar from the absence of a gizzard, of which traces appear to exist in other species of Microscolex; for in M. dubius Rosa speaks of " un ventriglio rudimentale, piatto, in forma di coppa; " as to the only other known form, Microscolex modestus, Rosa found that " il ventriglio esiste, m a cosi rudimentale da non potersene veder le traccie che nelle sezioni." It is not always possible to detect the presence or absence of a gizzard without having recourse to section cutting. Pontodrilus, for example, is stated by Perrier to be without this special region of the oesophagus ; but it is obviously present, though certainly much reduced, when the anterior region of the worm's body is examined by means of sections. The pharynx ends in the third or fourth segment, and, as in other Earthworms, there are masses of glands upon the dorsal surface. These glands, which seem to represent a part of the system of septal glands in the lower Oligochaeta, are not confined, in Microscolex algeriensis, to the pharyngeal region of the alimentary tract; they extend back as far as the ninth segment, and therefore suggest more clearly the septal glands, with which they must surely be homologous. It is interesting to recall the fact that these glands occur also in Ocnerodrilus, which is another form near to the border line between the terricolous and liinicolous Oligochaeta, though nearer to the latter than is Microscolex. The oesophagus of Microscolex algeriensis is divided into two regions; up to the end of segment vii. it is not markedly vascular, and the living epithelium is composed of more densely packed cells, which gives it a more deeply stained appearance under the microscope. The rest of the oesophagus has a richly-developed vascular network, and the epithelium appears to have a looser texture, the cells being less tightly packed; from this circumstance the posterior region of the oesophagus looks paler in sections. In the xvth segment the oesophagus becomes much narrower and then suddenly widens into the intestine which commences in the xvith segment. |