The segmentation is unequal, and would appear to be followed by an epibolic invagination. The outer layer of cells (epiblast) becomes covered with cilia, and the inner is transformed into a non-cellular (?) central yolk mass. At this stage the larva is hatched, and commences to lead a free existence. In the next stage observed by Metschnikoff, the mouth, oesophagus, stomach, and anus had become developed; and two pairs of feet were present. In both of these feet Chætopod-like setæ were present, which in the hinder pair were simple fine bristles without a terminal hook. The papilliform portion of the foot is at first undeveloped. The feet become successively added, like Chætopod segments, and the stomach does not become dendriform till the whole complement of feet (5 pairs) are present. In the primitive covering of cilia, combined with a subsequent indication of segments in the formation of the feet and setæ, the larva of the Myzostomea shews an approximation to the Chætopoda, and the group is probably to be regarded as an early Chætopod type specially modified in connection with its parasitic habits. BIBLIOGRAPHY. (379) L. Graff. _Das Genus Myzostoma._ Leipzig, 1877. (380) E. Metschnikoff. "Zur Entwicklungsgeschichte d. Myzostomum." _Zeit. f. wiss. Zool._, Vol. XVI. 1866. (381) C. Semper. "Z. Anat. u. Entwick. d. Gat. Myzostomum." _Zeit. f. wiss. Zool._, Vol. IX. 1858. GASTROTRICHA. A few observations of Ludwig on the winter eggs of Ichthydium larus shew that the segmentation is a total and apparently a regular one. It leads to the formation of a solid morula. The embryo has a ventral curvature, and the caudal forks are early formed as cuticular structures. By the time the embryo leaves the egg, it has almost reached the adult state. The ventral cilia arise some little time prior to the hatching. BIBLIOGRAPHY. (382) H. Ludwig. "Ueber die Ordnung Gastrotricha _Metschn_." _Zeit. f. wiss. Zool._, Vol. XXVI. 1876. CHAPTER XVI. NEMATELMINTHES AND ACANTHOCEPHALA. NEMATELMINTHES[153]. [153] The following classification of the Nematoda is employed in this chapter: { Ascaridæ. { Strongylidæ. I. Nematoidea. { Trichinidæ. { Filaridæ { Mermithidæ. { Anguillulidæ. II. Gordioidea. III. Chætosomoidea.