952 KECOED OF CURRENT RESEARCHES RELATING TO 



By the aid of high magnification, it is possible to see in the hinder 

 portion of the cephalic region a very delicate longitudinal canal — 

 the head-kidney ; this runs for the greater part of its course parallel 

 to the ventral longitudinal muscle, and opens ventrally at the anterior 

 end of the mesodermal band, where its lumen is continuous with a fine 

 pore in the ectoderm. Anteriorly, this excretory organ terminates in 

 a small solid swelling, which is distinguishable by its clearer 

 appearance from the dark granular protoplasm of the walls of the 

 canal. The termination would appear to be formed by a single cell, 

 and the rest of the canal by a very small number of cells. 



The point to be noted in the older examples of the same stage is 

 chiefly the great increase in the size of the trunk ; this affects 

 chiefly the mesodermal bands which, growing rapidly, get their cells 

 arranged in two, then in several rows, and, in time, in two layers. 

 Of other characters, the most important are the appearance of a 

 pre-anal circlet of cilia, not hitherto distinctly seen in the larva either 

 of Mollusca or Rotatoria ; the pre-oral circlet gradually becomes 

 reduced to one row ; the cells which form the inner layer of the 

 integument become considerably modified ; at first connected with 

 one another by numerous in*ocesses, they become in time converted 

 into a membrane, which forms an internal sac ; a secondary branch is 

 developed on the kidney, and in time the primary one is atrophied. 



The characters of the second period are shortly summed up in 

 saying that the increase in size is still chiefly seen in the region of 

 the trunk ; the mesodermal bands become further developed in the 

 characteristic Annelid mode ; starting from before backwards they 

 give rise to the primary segments. In these there appear cavities 

 which are due to the separation of the entero-muscular from the 

 dermo-muscular layer. It is at this period that the oesophageal 

 commissures and the lateral ganglia of the ventral cord begin to be 

 developed, and that we see the first appearance of the ventral 

 setigerous sacs ; these are placed in the first trunk-segment, and at 

 the sides of the ventral longitudinal muscles. 



In the third period the process of segmentation comes to an end, 

 and the separate segments all take very much the same appearance. 

 Metamerism is very clearly shown, internally, by the appearance of 

 segmental ciliary circlets, and (later on) by the peculiar arrange- 

 ment of the pigment. At the same time we find that the internal 

 dissepiments, which primitively divided the secondary coelom into 

 segmental cavities, are converted into filaments, and are gradually 

 replaced by a tissue of ramifying cells which extend between the 

 dermo-muscular and the entero-muscular plates. Nor can any very 

 distinct indications of segmentation be said to be afforded by the 

 ventral ganglionic cord. 



Turning to the development of the setfe, we find that their sacs 

 have been growing inwards towards the coelom, and transverse 

 growths give indications of the muscles of the setee ; internally a 

 small cavity is formed, and at its base there appears a small, highly 

 refractive corpuscle, which is the tip of the seta. This grows broader, 

 and elongates ; its cavity is still hollow, its chitiuous walls show signs 



