ANNULATA. 263 



lengtli of the body, and the number of segments, continued to in- 

 crease, the disk with its vibrating cilia (b) still existing. This disk 

 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 Bothriocephali, described in a former lecture. 

 Each ring originally consists of an upper ((/) and an under half- 

 ring (A), 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 beau- 

 tifully exemplify the repetition of structures and phenomena, charac- 

 teristic of mature animals widely separated in the natural scale, in 

 the immature states of an intermediate species. 



With regard to tiie genus Aphrodite, Sars * took a small species, 

 in the month of February, on the Norwegian coast, which produced 

 richly-coloured ova, and he traced the successive development of 

 these. He found that the ova escaped from small pores, and were 

 received into a kind of pouch beneath the dorsal scales, and there 

 underwent their development and escaped. He observed the geo- 

 metrical division of the rose-coloured yolk, and the clear spot in the 

 centre of each successive division. The embryo is an active loco- 

 motive oval mass, with a little group of cilia, and an indication of an 

 eye-speck to guide its course ; after swimming freely for twenty-four 

 hours the development of segments commenced. In the Cystonereis, 

 in which the ova are incubated in marsupial sacs, the development of 

 the animal appears, as in Koch's Eunice, to take place without that 

 amount of metamorphosis which has been observed in other Errantia. 

 First, there are two short tentacula, then three ; then the animal 

 elongates, and before it quits the marsupium, it presents a recog- 

 nizable miniature of the parent. 



Observations have been made on the development of the tubicolar 

 anellids by Milne Edwards. f He found a group of ferruginous yellow 

 ova, aggregated in a gelatinous mass, at the entry of the tube of the 

 Terebella ?iebulosa. The embryo, when excluded, is monadiform, 

 and swims by the vigorous vibrations of its superficial cilia; it 

 lengthens, and the cilia, which at first were generally diffused, 

 become confined to a cincture behind the head, a transverse ventral 

 band near the tail, and a small circle round that part. The head is 

 distinguished by two red eye-specks ; new segments are successively 

 added, one behind the other, and always in front of the anal one ; 

 but as yet the embryo is apodal. The tubercles and seta; are next 



* ecu. t ccvi. 



64 



