374 



Mr. W. H. Caldwell. On the 



| Dec. 7, 



The main vessels are two in number. 



The afferent vessel to the tentacles divides at the median dorsal 

 region of the septum. 



Each half passes into a vessel lying at the base of the tentacles. 

 From this caecal vessels pass into these. A second vessel lying out- 

 side the former is also by means of a valvular arrangement in com- 

 munication with the same ca3cal vessels in the tentacles. From the 

 outside ring on either side passes a lateral vessel to the ventral side of 

 the oesophagus, where, joining its fellow of the opposite side in the 

 left anterior division of the body-cavity, it runs as the single efferent 

 vessel to the hind end of the foot, giving off: numerous csecal vessels 

 in its course. 



Further, there is a sinus round the stomach. This arrangement 

 will be understood when its development is described below. 

 The walls of all the vessels are contractile. 



Generative System. 



The animals are hermaphrodite. Tho ova and spermatozoa are 

 formed from cells of the efferent blood-vessel, which runs in the left 

 anterior chamber of the body- cavity. Round this vessel lies the so- 

 called " fat body," which is composed of large cells developed on the 

 wall of the caecal prolongation of the blood-vessel. The ova and 

 testis lie in this tissue on opposite sides of the main vascular trunk. 

 Thus the nerve- cord and the generative cells are asymmetrically 

 placed. They lie on the left side of the foot. 



Development. 



The following is a brief summary of the more important points : — 



1. At the stage of four segmentation spheres, a division into two 

 smaller clear and two larger opaque cells indicates the future ectoderm 

 and endoderm. 



2. The segmentation proceeds with considerable regularity, and 

 results in a planula with half the cells smaller and less columnar 

 than the other half. 



3. Invagination of the larger cells almost obliterates the segmenta- 

 tion cavity, and a spherical gastrula with a blastopore is the final 

 result of invagination. The gastrula becomes oval by the growth 

 forwards of the ectoderm to form the praaoral lobe, and the blastopore 

 persists as the mouth. 



Themesoblast is formed bilaterally from the endoderm on either side 

 of the blastopore. From the time when two or three mesoblast cells 

 are budded off on either side a cavity is present in each mass so formed. 

 These cavities are the two halves of the body-cavity. I regard this 



