TTPE ANNELIDA. 215 



and continually adding to the bands by the formation of new 

 cells. The bands as growth proceeds break up into a number 

 of masses, the mesoblastic somites, in the interior of which 

 cavities appear, and adjacent pairs of masses growing dorsally 

 and veutrally finally come into contact above and below the 

 digestive tract, the dorsal and ventral mesenteries of the 

 intestine being thus formed, and later a metamerization of the 

 body-wall, corresponding with that of the mesoderm, also takes 

 place. The anterior cone of the larva, which at first sur- 

 passed in size the posterior one, gradually becomes smaller, 

 and the prototrochal cilia, and in some cases the cells also, are 

 thrown off. The apical plate takes part in the formation of 

 the supraoesophageal ganglion of the adult, and the lateral 

 nerve-cords arising from it form the circumcesophageal com- 

 missure, becoming connected with thickenings of the ventral 

 hypodermis arranged metamerically and representing the ven- 

 tral chain of ganglia. The head-kidneys gradually disappear, 

 being merely provisional larval structures, and new nephridia 

 of the Annelid type develop from the mesoderm of the trunk- 

 metameres. 



Although the Trochophore larva occurs in the life-history of many of 

 the Annelids, as well as in other groups as will be seen later, yet never- 

 theless it is not invariably present. In some forms a single band of large 

 cilia .runs around the middle of the body, which is elsewhere uniformly 

 ciliated, while in others the cylindrical larva is surrounded by several 

 bands of cilia succeeding one another at definite intervals. In certain 

 species the larva is provided with very long setae which are thrown off 

 during larval life, and are interesting on account of similar setae having 

 been found in fossil forms, though absent in recent adult species. 



It is also worthy of note that in some forms the Trochophore larva is 

 succeeded by a well-marked stage in which, in addition to the head seg- 

 ment, three trunk segments are developed. It is possible that this may 

 represent an ancestral form from which certain other groups have taken 

 their origin. 



In what may be considered exceptional cases a non-sexual repro- 

 duction by budding also occurs. In the genus Protnla a zone of growth 

 occurs in the sixteenth segment, and at this point later separation takes 

 place, a new head developing for the posterior individual from the original 

 seventeenth segment. In one species of Syllis the new individuals 

 arise not only in a linear series but also as lateral buds, so that a 

 branching colony is produced ramifying through the canal system of the 

 Hyalosponge in which the form lives. The buds eventually separate as 



