ANNELIDA. 71 



unite together, and the setigerous sacs as well a*s the nephridia 

 grow out from the band thus formed. 



Mesoderm. The whole of this is developed from the primary 

 mesoblasts (M). These lie behind the other teloblasts and give 

 rise to a mesodermal band (ms in K to 0) on each side which 

 forms the middle layer of the germinal bands. From the front 

 ends of these bands stellate cells bud off' in front and form a 

 mass above the mouth (see 0) by which the two bands are 

 connected together. Similar cells are budded off' in the trunk- 

 region, but these do not develop into any adult structures. 



The mesodermic bands gradually broaden and thicken, their 

 posterior ends being smallest because they have only recently been 

 derived from the teloblasts. In each band a series of cavities 

 (cce) appear from before backwards. These become the sections 

 of the adult coelom and indicate the position of the future seg- 

 ments. Where the bands unite an unpaired "head-cavity" (P) 

 is developed. The mesoderm which forms the outer boundary 

 of each cavity gives rise to part of the body-wall, that making 

 up the inner boundary originates part of the gut-wall, while the 

 partitions between adjacent cavities become septa. 



The mesodermal bands gradually get broader by the division 

 of their cells and unite together from before backwards, first 

 ventrally and then dorsally, making their way between the other 

 layers, and ultimately forming a continuous sheet of mesoderm. 

 The corresponding coelomic cavities of the two bands fuse com- 

 pletely together dorsally, but remain separated ventrally by a 

 thin mesentery. The blood-vessels are developed as spaces in 

 the mesoderm which at first (except in the case of the hearts) 

 have no proper walls of their own. The dorsal vessel has a 

 double origin, being formed from two lateral vessels (l.v in P) 

 which gradually shift upwards and fuse together. Each neph- 

 ridial funnel is formed by the subdivision of a large mesodermal 

 cell which soon becomes connected with an ingrowth from the 

 nephric band. 



The endoderm becomes the lining of the mid-gut, including 

 the epithelial part of the cesophageal pouches and calciferous 

 glands. 



Sexual Reproduction. 



There is some difficulty in comparing the reproduction of unicellular 

 animals (Protozoa) and multicellular animals (Metazoa), because in the 



