No. 3.] GROWTH AND METAMORPHOSIS OF TORN ARIA. 427 



have almost obliterated the lumen of the body cavity above 

 them. In the mid-ventral line the walls of the body cavities of 

 the two sides come into contact, and at the point of union is 

 left a large space — the cavity of the ventral blood-vessel (vv). 

 Around the nerve chord there are four dorsal extensions of this 

 (last) pair of body cavities. The upper pair are simple exten- 

 sions of the body cavities from the sides. The small pair 

 below the nerve chord at be 3 come also from the same pair of 

 body cavities, but arise back of the nerve chord, and extend for- 

 ward into the proboscis region. It is this pair of extensions 

 that go to form the dorsal blood-vessel that is to be seen just 

 beneath the nerve chord and bounded by the walls of the two 

 cavities. 



Fig. 65 passes through the second pair of gill pouches, which 

 have broken through to form a pair of gill slits. At the 

 external openings the ectoderm becomes continuous with the 

 endoderm. The inner openings of the gill slit into the digestive 

 tract are on each side richly ciliated. The small isolated block 

 in the upper part of the section contains the pair of extensions 

 from the body cavities (be 3 ), which enclose between them the 

 dorsal blood-vessel, and it is this pair that we saw in the last 

 section beneath the nervous system. The ectoderm in the 

 mid-dorsal line is thicker than elsewhere and contains a deeper 

 zone of nerve fibres. It is this ectoderm that is continuous 

 with the nerve chord of the last figure. In the lower part of 

 the section the body cavities (be 3 ) completely fill up the region 

 between the digestive tract and ectoderm. The cells of their 

 walls, especially in the ventral region, show a differentiation into 

 muscle fibre. The nucleus lies in the inner part, and the outer 

 shows a deeply staining portion which I think undoubtedly rep- 

 resents the contractile portion of the cell. It is thus seen that 

 the second pair of gill openings has been formed before the first 

 pair. Whether this is the rule I have no means of knowing, as 

 I did not have enough material to settle the point. Nor did I 

 have embryos old enough to show the origin of the collar pores 

 which form in the region of the first pair of gill pouches. 



A section through the posterior portion of the larva is shown 

 in Fig. 66. The ectoderm is not so thick as in the anterior 

 regions, and has almost no zone (except in the mid-dorsal line) 

 of nerve fibres. The digestive tract is swung by the mesen- 



