354 TUNICATA. 



primary body-cavity (Fig. 167, ms) which yields the blood-corpuscles, 

 the connective tissue, the body- musculature, as well as the genital and 

 excretory organs, while the posterior part gives rise to the larval 

 caudal musculature (Fi^. 168 B, m). 



In Distaplia, in which, after the closure of the blastopore, the 

 entoderm forms a solid cell-mass (Figs. 157, 164 A), the enteric 

 cavity arises only later through the shifting apart of these cells (Fig. 

 164 B). In this way a posterior part of the body is marked off; in 

 this the cells of the entoderm separate into the rudiment of the 

 chorda (Fig. 164, ch) and into that of the solid sub-chordal enteric- 

 process (c), while the large entoderm-cells in the anterior region of 

 the body (Fig. 165, en) mix later with the mesenchyme and probably 

 disintegrate. In other respects there is no essential difference be- 

 tween the development of Distaplia as described by Davidoff (No. 

 14) and that of other Ascidians. It should be pointed out that the 

 elements of the food-yolk here appear equally distributed in all the 

 tissues (the ectoderm, the mesoderm and the entoderm). 



D. Development of the free-swimming larva. 



External form of the body. We have already pointed out that, 

 at the time when the medullary tube develops, the embryo becomes 

 elongate and pear-shaped. In this way a broader anterior section is 

 marked off from a narrower posterior section (Fig. 162) which gives 

 rise to the tail of the larva. This latter part of the body next grows 

 greatly in length, less, as Seeligek points out, by the multiplication 

 than by the elongation of the cells composing it. At the same time, 

 it becomes more distinctly constricted from the anterior section and 

 curves round ventrally (Fig. 163). As the tail, which is now bent 

 downwards and forwards, increases in length, its posteiuor end not 

 only reaches the anterior end of the body but even grows upwards 

 again at the right side of the latter. In this process, the tail be- 

 comes twisted on its longitudinal axis, so that the nerve-tube appears 

 shifted to the left side of the embryo (Fig. 170, p. 368). 



The anterior region of the body, which at first appears more or less 

 spherical, lengthens later, and in the larva is ovate (Figs. 167 and 

 168). Three prominences, arising as thickenings of the ectoderm, 

 can soon be seen at its anterior end ; these are the rudiments of the 

 papillae for attachment (Figs. 167, 168, 170, h), through which, by 

 means of a secretion yielded by the glandular epithelium, the fixation 

 of the larva takes place. 



