SOME PROBLEMS OF ANNELID MORPHOLOGY. 6l 



almost like a bi-convex lens, with a circle of powerful 

 cilia (prototroch) running around the edge, a mouth at 

 one point near the edge, a retort-shaped alimentary canal 

 (dotted in the figure) and a brain (cerebral ganglion) at 

 the upper pole. The trochophores of other annelids 

 have the same general structure as this, but are often 

 more rounded in form, sometimes being nearly or quite 

 spherical, with the prototroch at the equator. The anus 

 is always at the opposite pole from the brain. 



The subsequent history of this larva reveals the sin- 

 gular fact that the parts thus far described become, 

 almost entirely, converted into the Z^^^^^aT or prostomium 

 of the adult. The trunk is a later formation, growing 

 down from the lower pole of the larva — like a bud, as 

 it were — and becoming divided into segments. (Fig. 2, 

 B and C). At the posterior (lower) end of the body there 

 is a kind of growing point, like the terminal bud of a 

 plant, at which rapid cell-formation takes place, so that 

 the tip is carried further and further down and the body 

 steadily elongates in one direction. The segments are 

 formed successively, those in front being the oldest 

 while new segments are continually in process of forma- 

 tion, one after another, at the growing point. This is a 

 typical case of apical or unipolar growth. Examining 

 the structure of the growing point more narrowly we find 

 that the internal tissues (mesoblast) are arranged in two 

 widely separated lateral bands, which, as the trunk grows 

 older, widen out and grow together along the median 

 line, ultimately giving rise to muscles, blood-vessels, ex- 

 cretory organs, reproductive organs, etc. This process 

 is a form of concrescence ; but this mode of growth will 

 be seen more clearly in the development of the leech, 



