268 EMBRYOLOGY 



(perianal) circle of cilia, which is situated on the posterior 

 portion of t!ie body (Fig. 118 B) ; this is lacking in many- 

 other Annelids. Furthermore there is added on the ventral 

 side a ciliated area extending from the mouth backward, 

 the adoral ciliated zone (Figs. 118 B and 128 A, p. 280). 



The metamorphosis of the Trochophore larva into 

 the adult worm will be followed in Polygordius, a form in 

 which it takes place in a particularly clear manner. The 

 Polygordius larva was first discovered by Loven, and held 

 to be that of a cha3tiferous worm. Ant. Schneider showed 

 that Loy^n's larva belonged to Polygordius. It has the form 

 of a typical Trochophore (Fig. 119). The ciliate apparatus, 

 which encircles the larva at its greatest diameter, is com- 

 posed of two rings, one in front and one behind the mouth. 

 The preoral ring consists of a double, the post-oral of a single, 

 row of cilia. A third ring, the perianal ciliated band, makes 

 its appearance at the posterior end of the larva (Fig. 120 J5), 

 bat it is not formed until the later stages of development. 

 The development of the Trochophore into the perfect worm, 

 which has been thoroughly studied by Hatschek, consists first 

 of all in a growing out of the posterior section of its body 

 and a gradual reduction of the anterior part. 



At first a segmentation of the larva is noticeable (Fig. 120 

 A), which depends upon a marked change in the two meso- 

 dermal bands. These, which at first consisted of only a few 

 cells, have become by active cell-proliferation much more 

 voluminous. Each of them is separated into two cell-layers 

 (Fig. 133 A to C, p. 290), and spreads out toward the ventral 

 and dorsal lines. Then a segmentation makes its appear- 

 ance in them, proceeding from in front backwards (Fig. 120 

 A), and at the same time the two layers of the bands sepa- 

 rate from each other by the formation of a cavity in each 

 segment. In this way the primitive segments arise, the 

 outer and inner walls of which become in each segment of 

 the worm the somatic and splanchnic layers of the mesoderm, 

 and the walls, abutting on one another, form the segmental 

 boundaries (dissepiments) of the body of the worm.^ Since 



^ A more detailed description of these conditions will be found in the 

 discussion of the formation of the body cavity (comp. p. 289). 



