Alimentary Canal.—The pore at the tip of the proboscis leads into a short thin-walled tube, which is rarely evaginated; into the base of this tube projects the short bluntly conical apex of the large ovoid muscular pharynx (or gullet?); this is lined by an epithelial layer of nucleated protoplasm, which secretes a strong cuticle. The stomach is a wide tube, somewhat dilated in each segment between the paired dorsi-ventral muscles, and tapering behind to end in the terminal anus. Four minute glands open at the junction of the pharynx and stomach.
Kidneys.—These are a pair of blind pear-shaped sacs, ciliated within (the only case of ciliation in Echinoderes), lying in the eighth segment, and opening by the taper ends right and left on the back of the ninth segment.
Nervous System.—All that has been clearly defined of this is a small brain or ganglion lying dorsally at the junction of the pharynx and stomach. From two to eight eye-spots have been described by earlier writers, but Reinhard was unable to find them in the (distinct) species which he principally worked at, though he noted their existence in the solitary specimen of the original species, E. dujardini, which he obtained.
Reproductive Organs.—The sexes are distinct. The reproductive glands form a pair of tubular sacs, opening ventrally on either side of the anus, and extending forwards beside the gut as far forwards as the fifth to the second segment in the male, but only to the fourth at furthest in the female. The ova are large nucleated cells embedded in the protoplasmic lining of the ovarian sac, and acquiring a distinct shell as they approach its opening. Three-quarters of the testis sac is occupied with granular protoplasm containing a quantity of small nuclei; the lower part alone contains mature spermatozoa. Adjoining each external opening in the male are a pair of short hollowed spines, which may perhaps serve as organs of copulation; but nothing is really known of this process or of the development of the egg. It is almost certain, from the absence of developing eggs within Echinoderes, that the genus is not viviparous.
From the foregoing description it is obvious that Echinoderes approaches the Nematoda very closely: the two main points of difference are its ciliated kidneys and its bilaterally paired sexual organs. Possibly the study of such forms as Desmoscolex (Fig. 81, p. [159]) may reveal closer affinities.
[Zelinka (Verh. D. Zool. Ges., 1894 and 1898), has given a preliminary account of a new research on this group. The principal addition is the discovery of a ventral nerve-cord, with a ganglionic dilatation in each segment, lying in the ectoderm of the body-wall, as indeed do the brain and nerve-collar. He divides the genus into two Orders according as the orifice of the retracted fore-part of the body is slit-like or circular. The former (Homalorhagae) retract the first two segments with the proboscis; they are mud-dwellers, sluggish, eyeless: the latter group (Cyclorhagae) only retract the first segment with the proboscis; they crawl among algae, and mostly have paired pigmented eye-spots, each with a lens, imbedded in the brain.—M. H., Jan. 1901.]
ARCHIANNELIDA, POLYCHAETA, AND MYZOSTOMARIA
BY
W. BLAXLAND BENHAM, D.Sc. (Lond.), Hon. M.A. (Oxon.)