xiv CHAETOGNATHA 433 



columnar in this region and their nuclei retreat to their bases, where 

 they undergo rapid division and give rise to small rounded ganglion 

 cells. This proliferation does not take place evenly all over the 

 ventral surface but along two parallel lines, and we are involuntarily 

 reminded of the double nature of the Annelid ventral nerve cord. 

 Sparsely scattered over the body are groups of sensory hairs, arranged 

 like fans, transversely to the long axis of the body. These are more 

 numerous on the head. At their bases groups of sensory nuclei can 

 be seen. 



At this stage the young Sagitta is hatched. If the foregoing 

 description has been followed, it will be seen that it is a solid worm 

 very unlike the adult, and therefore merits the name of larva. It is 

 about 1 mm. long, and it swims near the surface of the water, 

 propelling itself by lateral jerks of the tail fin, which is already 

 developed. At the bases of the mesoderrn cells in the trunk and tail 

 the fine refractive muscular fibrils can already be made out. They 

 all run longitudinally. 



After swimming for about two days the larva undergoes a com- 

 paratively rapid change which makes it much more nearly like the 

 adult in structure. Cavities suddenly reappear both in the 

 mesoderm of the trunk and in that of the head, and so the larva 

 becomes inflated and more transparent. 



The cephalic hood becomes a prominent fold, and under it a 

 pair of hooks is to be seen on each side. These hooks gradually 

 increase in number by the addition of new ones in front, till the full 

 number characteristic of the species is attained. At each side of the 

 mouth a rounded ectodermic thickening appears, which is the so-called 

 lateral ganglion. The eyes also appear as minute black specks. The 

 cavities in the head disappear again, and the head mesodermal cells 

 become arranged in definite bundles, forming muscles for moving the 

 hooks. 



As the larva grows in length the ventral ganglion becomes 

 relatively shorter and the ectoderm cells in front and behind it 

 become vacuolated so as to form a kind of floating tissue (vac, Fig. 

 341). Such vacuolated cells are found all over the trunk through- 

 out life in Spadella draco, but of course they disappear later in life 

 in Sagitta. 



In the tail region the tail fin becomes large and prominent and 

 extends a good distance forward. It is separated by a gap from two 

 lateral fins which have appeared, one on each side. In the gap on 

 each side is situated a large tactile organ, borne on an ectodermal 

 prominence (sens, Fig. 341). 



On the fourth day the genital cells, which up till now have been 

 disposed in two pairs, an anterior and a posterior pair on each side, begin 

 to migrate across the body-cavity from its splanchnic to its somatic wall. 

 As this takes place a lateral septum is formed, cutting the coelom on 

 each side into trunk and tail portions, and separating the anterior 

 genital cells on each side, which are the rudiments of the ovaries, 

 VOL. i 2 F 



