No. 3.] GROWTH AND METAMORPHOSIS OF TORNARIA. 425 



is another pair of body cavities {bc z ) enclosing a large space 

 between them (dv). These {be 3 ) are the anterior extensions of 

 the last pair of body cavities and bound an anterior extension 

 of the dorsal blood-vessel {dv), which runs into the proboscis and 

 joins subsequently the proboscis blood-vessel (heart). 



A section more anterior to the last, passing through the base 

 of the proboscis and the anterior edge of the collar, is shown by 

 Fig. 59. The section cuts the collar below and at the sides, 

 but just anterior to the collar above. The collar ectoderm is 

 here thick as in the last section. In the upper part it turns 

 back at the sides to become continuous with the ectoderm of the 

 proboscis. Over the proximal end of the proboscis (just an- 

 terior to the mouth), particularly on the upper side, near wp, the 

 ectoderm is greatly thickened, having a broad zone of the nerve- 

 fibre layer. It is this part which is directly continuous with the 

 lower part of the nervous system of the last figure. The exter- 

 nal opening of the anterior enteroccel (the water pore) pierces 

 this thickened ectoderm on the dorsal surface and a little to the 

 left of the middle line at wp. Beneath the ectoderm, in the 

 upper part of the proboscis and above the digestive tract, is a 

 mass of somewhat spongy tissue. There are several distinct 

 structures here. At {dv) is the anterior end of the dorsal 

 blood-vessel. The section passes through the point at which it 

 fuses with the proboscis blood-vessel, and the anastomosing cells 

 across the lumen seem to break the large vessel up into smaller 

 ones. The rest of this area is filled by portions of the body 

 cavities. The digestive tract is divided into two portions, an 

 upper and a lower. The separation is exactly the same as in 

 the preceding case. In the folds are seen the chitin-like rods. 

 The upper division passes through the plane in which the dorsal 

 division of the digestive tract becomes continuous with the 

 notochord {nc), and the cells already have assumed the peculiari- 

 ties of the notochordal tissue. Parts of the anterior ends of the 

 collar body cavities fill the space between the digestive tract and 

 the ectoderm. 



A section just anterior to the last is shown in Fig. 60, passing 

 also through the base of the proboscis. Over the greater part 

 of the section the ectoderm is greatly thickened, but on the mid- 

 ventral line it is thin and passes here into the mouth. The 

 centre of the section contains several important organs. In the 



