306 MYZOSTOMEA. 



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 Chretopod-like setee 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 be- 

 come successively added, like Chsetopod 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 indica- 

 tion of segments in the formation of the feet and setse, the larva of the 

 Myzostomea shews an approximation to the Chaetopoda, and the group 

 is probably to be regarded as an early Chsetopod type specially modified 

 in connection with its parasitic habits. 



BlBLIOGKAPHY. 



(379) L. Graff. Das Genus Myzostorna. Leipzig, 1877. 



(380) E. Metschuikoff. " Zur Eutwicklungsgeschichte d. Myzostomum." Zeit. 

 f. wiss. ZooL, Vol. xvi. 1866. 



(381) C. Semper. " Z. Anat. u. Entwick. d. Gat. Myzostomum." Zeit. f. iviss. 

 Zoo/,, Vol. ix. 1858. 



GASTROTRICHA. 



A few observations of Ludwig on the winter eggs of Ichthyditim 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 Gastrotriclm Metschn." Zeit. f. wiss. ZooL, 

 Vol. xxvi. 1876. 



