THE CH.ETOGNATHA. 449 



sac-like diverticula on either side, m.x. In the next stage, G, the mesoderm has 

 divided into two pairs of mesodermic chambers, one for the head, c.m.s., and one 

 for the trunk, th.ms. The anterior end of the primitive gut unites with an ecto- 

 dermic infolding at the cephalic apex, that gives rise to the mouth and stomodasum, 

 n.s.t.', the posterior end extends backward and ultimately unites with the caudal 

 apex at the point where the anus is formed later. Meantime the two primitive 

 germ cells have divided into four cells, two on either side of the free end of the 

 enteron. The two anterior ones are the anlagen of the ovaries, or., and the two 

 posterior ones of the testis, /. 



In the next stage, H, the mesocceles break down, the mesoderm forms a practi- 

 cally solid mass, and the post-anal section of the body develops. During the early 

 cleavage stages, a large nucleus appears in association with the cells that give rise 

 to the mesoblasts and germ cells, C, x. Later it is transferred to the primitive 

 germ cells, and finally breaks up into fragments and disappears. It appears to 

 represent the "yolk nuclei" so characteristic of arthropod eggs containing a large 

 amount of yolk. 



In the newly hatched young, /, three regions may be recognized, the head, 

 Pr.c., consisting of what appear to represent the rudiments of one, possibly three 

 pairs of appendages. It is partly enclosed by two lateral folds, mt.t, that may be 

 regarded as the remnants of a bivalve shell of the naupula. 



The trunk, Br.c., contains very large, paired ventral ganglia, ni.br., and the 

 ovaries, ov. A membranous partition separates the trunk from the caudal region, 

 a.b., in w T hich are located the testes, /. 



The three divisions of the body indicated at this early period are not com- 

 parable with metameres; they represent three tagmas, or three imperfectly devel- 

 oped body regions, head, thorax, and abdomen, such as are commonly seen in many 

 primitive arthropods, or in the embryonic stages of the more highly developed 

 forms. In the chaetognatha, either these regions have never been definitely divided 

 into metameres, or if so, the metameres have disappeared through the regressive 

 Y>r degenerative processes that are so prevalent in the acraniates. 



The Adult. The newly hatched young, without any strongly marked larval 

 stage, pass directly into the adult form. The most noteworthy fact in their devel- 

 opment is the relatively enormous size of the ventral ganglion in the younger, as 

 compared with the older stages, indicating a certain amount of degeneration in 

 the history of the group. 



The integument may be very thick and surprisingly complex for animals of 

 such a low grade of development. It consists, in some genera, of a complicated 

 irregular network of interlacing trabeculae, recalling the vacuolated or cancellous 

 ectoderm of the cirripeds, branchiopods, and tunicates. 



Excretory organs are not definitely known to occur, although the ducts opening 

 near the base of the mandibles, d, may be connected with such organs. They are 

 regarded by Moltchanoff as the nephridia-like ducts of the first metamere. 



There is no heart, and no circulatory organs, and there are no indications of 

 29 



