CHAP, vii ANNELIDA 147 



the trunk blastema; this cell buds so as to form a string of cells, which 

 become hollowed out so as to constitute a tube. 



Further cell multiplication has also been proceeding in the region 

 of the rosette or " Annelidan cross," around the apical plate, and this 

 region of new-formed cells is termed the head blastema. On each 

 side, just as in Planocera (see page 110), cells are budded inwards 

 into the blastocoele and form the rudiments of the larval cerebral 

 ganglion. Other cells of the larval mesenchyme form muscular 

 strands which reach from the apical plate to the sides of the 

 oesophagus and to the ventral ectoderm in front. 



As the post-trochal outgrowth of the body increases in length its 

 tissues begin to undergo histological differentiation, but unfortunately 

 the details of this have not yet been worked out in Polygordius. As 

 more and more somites are formed we can make out, in the most 

 anterior and advanced somites, the longitudinal muscles. These 

 appear to be laid down as hard refractive fibrillae in the basal 

 portions of the cells forming the outer wall of the coelom ; the visceral 

 muscles also make their appearance as similar fibrillae in the basal 

 parts of the cells forming the inner wall of the coelom. 



The origin of the posterior or permanent nephridia, the meta- 

 nephridia, as we may term them, has not been fully worked out. 

 Woltereck asserts that they arise as strings of cells, growing from 

 " pore-cells " situated in the ectoderm of the trunk blastema. At an 

 early stage of their development they appear as small packets of cells 

 lying in the outer wall of the coelom, where the insertion of the 

 oblique septum separates dorsal and ventral bundles of longitudinal 

 muscles. In the adult, as is well known, the inner ends of these 

 nephridia communicate with the coelom by ciliated funnels. How 

 these are formed has not been made out in Polygordius. 



The origin of nephridia in Annelida generally has been the 

 subject of much dispute, but a great deal of light has been thrown on 

 this subject by the results of investigations on the development of 

 the Oligochaete worm Oriodrilus, details of which will be given later. 



The head blastema undergoes important changes as the larva 

 grows older. We have already seen that underneath the apical tuft 

 of cilia there is formed a mass of nerve cells and nerve fibres, which 

 constitute the larval cerebral ganglion. From this ganglion there 

 proceeds eight radial nerves which go to the cells forming the 

 prototroch. Two of these, termed lateral nerves, are stronger than 

 the rest ; these cross the prototroch and join the ectoderm cells which 

 will give rise to the ventral nerve cord of the adult. They con- 

 stitute the nerve collar of the adult. Of the six other radial nerves, 

 two are situated on the anterior side of the larva, whilst four are on 

 the posterior side. 



Besides these radial nerves there is a network of ganglion cells 

 and fibres underlying the ectoderm of the upper half of the larva, 

 and, though in a much sparser condition, beneath a good deal of the 

 ectoderm of the lower half. There is also a ring of nerve fibres 



