ANELLATA. 147 



Sea, a discoid animalcule {fig. 17.), which rapidly moved by means 

 of two rows of vibratile cilia : the principal row 

 being situated upon a projecting ring (b), at the 

 margin of the disc. This ciliated body differed from 

 the gemmules of the Polypi, in being provided with 

 a mouth (a) and an anus, the latter occupying the 

 apex of the cone (c). The course of the alimentary 

 canal (c?), which extended from one to the other 

 aperture, was detected by feeding the little ani- 

 mal with indigo. In a short time the cone began 

 to elongate and to be divided into segments, which 

 were developed in four parts, the two principal 



Embryo of Nereis. i r. . iV, 



pieces forming halt-rings, one upon the upper, the 

 other upon the lower surface, which were united by two shorter lateral 

 pieces. Coincident with the elongation and segmentation of the body, 

 was the development of the head from the discoid surface (e), upon 

 which first the black ocelli, and then two pointed filaments, or an- 

 tennte (/), made their appeai-ance. The length of the body, and 

 the number of segments, continued to increase, the disc with its 

 vibrating cilia still existing. This disc is afterwards reduced to an 

 appendage on each side of the head, and finally disappears. The 

 new rings are added in front of, and not behind, the older ones, 

 agreeably with the order of development of the segments in the JBo- 

 thriocephali, described in a former lecture. Each ring originally 

 consists of an upper (ff) and an under half ring (/t), analogous to 

 the tergum and sternum in the external skeletons of Insects. The 

 tubular and setigerous feet are lastly developed from the small lateral 

 pieces. These observations beautifully exemplify the repetition of 

 structures and phenomena, characteristic of mature animals widely 

 separated in the natural scale, in the immature states of an interme- 

 diate species. 



LECTURE XIII. 



EPIZOA AND CIRRIPEDIA. 



The naturalist has often been baffled or led astray in his attempts to 

 discover the real nature and affinities of an animal by investigations 

 limited to the structure and habits of such animal in its mature state. 

 There are some species which undergo such extraordinary metamor- 



L 2 



