ANNELIDA 291 



raasculafcure its sesfmenfcal oriofin can be recosfnize 1 even in 

 the fully developed animal. The peritoneal epithelium is 

 also derived from the primitive se^rments. 



The splanchnic layer of the mesoderm produces so much 

 of the wall of the intestine as is not of entodermal or of 

 ectodermal origin, and the walls of the vessels also arise 

 from it. According to Salensky, the formation of the blood- 

 vascular system begins (in Psygmohranchus and Terebella) 

 in the form oE canals, which lie between the entoderm and 

 the splanchnic layer, and which are therefore really parts of 

 the segmentation cavity. Later these blood-filled cavities 

 are surrounded with a cellular wall, which comes from the 

 splanchnic layer. According to Kowalevsky's observations 

 also, mesodermal cells, which collect between the ectoderm 

 and splanchnic layer, form the walls of the vessels ; more- 

 over, the dorsal vessel (^Lumhricus and Griodrilus, according 

 to Vejdovsky) arises from separate paired fundaments. 

 These extend along the boundaries of the mesodermal 

 bands as they grow dorsad, and, advancing with them, finally 

 fuse with each other to form the dorsal vessel. This con- 

 dition is of especial interest from the fact that in Fleurochceta 

 (Megascolex) the separate fundaments of the dorsal vessel, 

 are retained in certain parts of the body throughout life 

 (Beddard). 



It appears questionable whether the cephalic cavity is formed in the 

 same way as the segmental cavities of the body, or whether it is to be 

 distinguished from these. On the first assumption, the two most anterior 

 primitive segments would unite for its formation, andtherefore outgrowths 

 of the mesodermal bands must have crowded forward past the oesophagus 

 even into the head region. The outer wall of the head and the muscula- 

 ture of the oesophagus would then be formed from the somatic and 

 splanchnic layers respectively in the usual way. The conditions were 

 described in this way by Kleinenberg in his earlier work, and Vejdovsky 

 also derives the cephalic cavity from the two " anterior ends " of the 

 mesodermal bands, which he describes as being fused (RhynchelmiH). 

 Opposed to this is the view, supported especially by the free-living larvte, 

 that the cephalic cavity arises by means of a separation of the two 

 primary germ-layers and by an immigration of mesodermal elements 

 from the trunk (Hatschek). According to this explanation, the first 

 pair of primitive segments lies behind the head, and the mesoderm of 



