THE ARTIIROPODA. 121 



The so-called closed tracheal system of many aquatic larvae, e.<j., 

 those of the Epliemeridae, as well as the tracheal gills connected 

 with it, are to be regarded as a form of respiratory organ secondarily 

 acquired in adaptation to life in water. 



The fore- and hind-guts arise in the Arthropoda as ectodermal 

 invaginations, the stomodaeum and the proctodaeum. The excretory 

 tubes known as Malpighian vessels found in the Myriopoda and 

 Insecta are diverticula of the proctodaeum. The same name is 

 given to similar blind tubular appendages of the intestine in the 

 Arachnida, but, since ontogeny makes it probable that the latter 

 belong to the enteron and are thus not of ectodermal but of ento- 

 dermal nature, they ought not to be homologised with the Malpighian 

 vessels. Tubular appendages similar in structure and function are, 

 on the other hand, found at the end of the enteron in the Crustacea 

 (Amphipoda), but these most probably must be regarded merely as 

 analogous structures. 



The phylogenetic origin of the Malpighian vessels is still obscure. It has 

 been thought that they might be true nephridia, which have become connected 

 with the proctodaeum, since structures resembling the nephridia are in some 

 Annelids found connected with the intestine (Nos. 2 and 9). In one case, 

 that of the Mcgascolides examined by Spencer, these glandular tubes, 

 which in structure are extraordinarily like nephridia, are, indeed, connected 

 with the stomodaeum, while, in Acanthodrilus, Beddard found similar struc- 

 tures connected with the proctodaeum. Just as, in Peripatus, nephridia have 

 been found drawn into the buccal cavity as salivary glands, so we might suppose 

 nephridia drawn into the proctodaeum, a process which is perhaps more probable 

 a priori than the former, since the nephridia in this case retain their original 

 function. Against such a view we have the ectodermal or entodermal origin 

 of these excretory tubes, and this is of all the greater weight since, according to 

 recent researches, the nephridia arise altogether from the mesoderm, and it 

 would therefore be impossible to imagine a persistence and a specially strong 

 development of the ectodermal portion simultaneously with a complete degenera- 

 tion of the mesodermal (inner) section.* 



The formation of excretory tubes starting from the intestine, such as has been 

 observed in the Amphipoda, is a noteworthy indication of the fact that parts of 

 the intestine are capable of taking over the function formerly carried on by the 

 nephridia. Even in the JVauplius we find a part of the intestine utilised for 

 excretion, the cells filled with urinary concretion forming slight swellings 

 (Vol. ii., Fig. 89, ds, p. 191). When these parts are transformed into caeca or 

 lengthen out like tubes, we have the excretory tubes of the Amphipoda 

 (eventually also of the Arachnida) or the Malpighian vessels of the Myriopoda 

 and Insecta, according as, in each case, the process takes place in the enteron or 



* [Some recent observers (Goodrich, Quart. Jonm. Micro. Sci., Vol. XXXVII., 

 1895, and Meissenheimer, Zeitschr. f. Wiss. Zool., Bd. lxiii., 1898), suggest 

 that the primitive nephridia may be largely if not wholly ectodermal, and 

 would distinguish these from the genital ducts and certain secondary nephridia, 

 which arise from the mesoderm as coelomic funnels. — Ed.] 



