258 INVERTEBRATA CHAP. 



start at the sides of the stomodaeum; they are indeed continuous 

 here with thickenings in the procephalic regions which form the brain, 

 and they continue backwards till they join one another behind the 

 proctodaeum. Thus the central nervous system of Donacia, like that 

 of Peripatus and ofAgelena, may be regarded as a long drawn-out loop. 

 The ridges are separated from one another by a shallow groove, the 

 neural groove, which reaches from the hinder end of the stomodaeum 

 back to the proctodaeum. 



As the appendages make their appearance the neural ridges 

 become segmented into thickenings lying at the bases of the append- 

 ages ; these form the ganglia of the ventral nerve cord. Successive 

 ganglia are, of course, not completely separated from one another, but 

 are united by thinner parts of the neural ridge, which are the 

 rudiments of the commissures. The inner parts of the ganglionic 

 rudiments soon separate from the outer larger parts, and the inner 

 form the definitive ganglia whilst the outermost layer forms ordinary 

 ectoderm. In the rudiment of the ganglion are to be seen a number 

 of rounded cells undergoing division. These are neuroblasts (n.b, 

 Fig. 204), and the cells resulting from their division are the neurons 

 or nerve cells. 



The ectoderm lining the neural groove also becomes two layered, 

 and the inner layer becomes separated from the outer and forms a 

 peculiar median string of cells, which also enters into the formation 

 of the ventral nerve cord. Each lateral thickening gives rise to a 

 lateral group of large pale nerve cells, on the dorsal surface of which 

 fibrillar substance appears. 



The median string of cells gives rise to three smaller groups of 

 ganglion cells, of which two arise from that portion of the string 

 which is at the level of the ganglion, and one from the inter-ganglionic 

 part of the median string. By the multiplication of the nerve cells, 

 the fibrillar substance becomes completely surrounded by them. 



In Donacia no less than twenty pairs of ganglia make their 

 appearance, three pairs in the head, three in the jaw region, three in 

 the thoracic region, and eleven in the abdomen. The first three, named 

 appropriately protocerebrum, deuterocerebrum, and tritocerebrum, 

 fuse together to form the brain ; the next three unite to form the 

 suboesophageal ganglion, and the three last ganglia of the abdomen 

 unite to form a large abdominal ganglion. 



Shortly after the appearance of the nerve ganglia the rudiments 

 of the tracheae make their appearance as ectodermal in vagi nations, 

 lying outside the rudiments of the ganglia and of the appendages, 

 when these are present. Eleven pairs of these invaginations make 

 their appearance on the last two segments of the thorax and on the 

 first nine segments of the abdomen ; they give rise to oval sacs which 

 unite with one another in a longitudinal direction, and result in the 

 great lateral tracheal stems of which the other trachea are outgrowths. 

 The last pair of the tracheal rudiments are larger than the rest, and 

 the ectoderm cells surrounding their openings (stigmata) are 



