200 SEGMENTED WORMS OR ANNELIDA. 



nally, opening on the head. These are known as "provisional nephridia " 

 or "head kidneys." They degenerate as the permanent excretory 

 organs develop, (c) The numerous permanent nephridia are for the 

 most part ectodermic, arising from the rows of nephridial cells already 

 described. Two parts of each nephridium, however, have a meso- 

 blastic origin, viz., the innermost part or the ciliated funnel, and the 

 peritoneal investment which ensheaths the whole organ. 



By the invagination of the blastosphere, a globular gastrula cavity is 

 formed. This forms the archenteron, the future mid gut, and elon- 

 gates with the growth of the embryo. To the completion of the entire 

 alimentary canal, however, two other processes are necessary, an in- 

 tucking of ectoderm from in front the stomatodaum or "fore gut " 

 which pushes the archenteron backwards and forms the future pharynx, 

 and a similar in-tucking of ectoderm from behind the proctod&um or 

 <; hind gut" which meets and fuses with the archenteron, and forms 

 the anus and a small portion of the posterior gut. 



The mesoderm begins with the two primary mesoblasts already 

 described. These multiply and form mesoderm bands, which, insinua- 

 ting themselves between ectoderm and endoderm, proceed to surround 

 the gut. At the same time, some of the mesoderm cells become 

 migratory, wander on to the head, and also surround the gut, before the 

 final trunk musculature is completed. The migratory mesoblasts of the 

 trunk appear to form a special larval musculature precociously devel- 

 oped, in order to enable the embryo to manage the enormous mass of 

 albumen (absorbed from the capsule) with which its body is distended. 

 The mesoderm bands grow in strength, and form a complete ring 

 encircling the archenteron. 



Origin of the body cavity. The mesoderm bands, growing in strength, 

 become tw r o layered. These two layers separate, the inner (splanchnic) 

 cleaving to the gut, the outer (somatic) clinging to the body wall. The 

 space between them is the body cavity or cctlome. But as the separa- 

 tion of somatic and splanchnic layers takes place, partitions are also ' 

 formed transversely, to become the septa which partition off the body 

 cavity into a series of segments. The cavity of the pre-oral segment or 

 prostomium differs somewhat from that of the others, being from the 

 first unpaired, instead of including two lateral cavities one on each side 

 of the gut. 



As to the blood vessels, the ventral or sub-intestinal appears first, as a 

 space between the wall of the archenteron and the underlying meso- 

 derm ; the dorsal vessel has a double origin, arising from the fusion of 

 two lateral vessels which develop like the ventral. The important 

 point is, that the blood vessels are at first long lacunar spaces, which 

 gradually acquire definite walls. By and by the " hearts " and other 

 complications in the vascular system appear. 



The reprodtictive organs, though probably arising from cells which 

 have kept to some extent apart from the formation of the embryo, 

 certainly appear in association with the mesoderm. 



The above is the account of the development of Lumbricus given by 

 Wilson ; another investigator, Bergh, differs from Wilson in several 

 important points. First, with regard to the nomenclature of the con- 

 stituents of the germ bands. 



