226 ABTHROPODA [CII. 



are interesting structures, since their development has shown that in 

 origin they are homologous with the excretory organs. The pharynx 

 leads by a short oesophagus into a roomy endodermic stomach which 

 reaches back nearly to the anus, a short proctodaeum only being 

 interposed (Fig. 93). 



The structure of the heart and pericardium closely resembles 

 that of the same organs in Myriapods and Insects. 



The animal breathes by bunches of tracheae or short tubes 

 which pass from the exterior into the tissues and convey air. Their 

 external openings or stigmata are partly in two rows above and 

 between the legs and partly scattered irregularly. 



At the base of each leg is an excretory organ which ends in- 

 ternally in a vesicle. Embryological research has shown that this 

 vesicle is a remnant of the true coelom which is spacious in the 

 embryo, but becomes displaced as development proceeds by the 

 haemocoele. Enclosed in the proximal part of the leg there is a 

 gland called the crural gland. 



Peripatus is bisexual, and again embryology has demonstrated 

 that the cavity of the sexual organs is coelomic. The male deposits 

 its spermatozoa in packets in the body of the female. It is not 

 known how they reach the ova but they are usually found in the 

 ovary and possibly bore their way through the tissues, as they do 

 in some leeches. The ducts of the reproductive organs are, like the 

 excretory organs, coelomiducts and belong to the same metameric 

 series as the excretory organs. 



The nervous system consists of a brain and two ventral cords, 

 which however do not approach one another but lie wide apart. 

 They are connected by nine or ten transverse commissures in each 

 segment (Fig. 93). Posteriorly the two ventral nerve-cords fuse 

 above the proctodaeum, an arrangement which recalls what occurs 

 in certain primitive Mollusca. 



There are many species of Peripatus, which are by some 

 authorities grouped into three or four genera. They are found in 

 widely separated parts of the world and afford, as is often the case 

 with archaic animals, an excellent example of " discontinuous 

 distribution." They have been found in South America and the 

 West Indies, in South Africa, in Australia, New Zealand and in 

 some of the islands of the Malay Archipelago and in Lower Siam. 



The development of Peripatus first definitely solved the problem 

 of the nature of the various spaces in the Arthropod body and has 

 also thrown much light on some of the peculiarities of insect 



