DEVELOPMENT AFFINITIES 



483 



The segmentation of the egg in M. macronyx is total and 

 equal, according to the observations of von Erlanger. 1 A blastula, 

 followed by a gastrula, is formed. The blastopore closes, but 

 later the anus appears at the 

 same spot. There are four pairs 

 of mesodermic diverticula which 

 give rise to the coelom and the 

 chief muscles. The reproductive 

 organs arise as an unpaired 

 diverticulum of the alimentary 

 canal, which also gives origin 

 to the Malpighian tubules. The 

 development is thus very primi- 

 tive and simple, and affords no 

 evidence of degeneration. 



With regard to their position 

 in the animal kingdom, writers 

 on the Tardigrada are by no 



1 r\ -r< n/r--n FIG. 254. Male reproductive organs of 



means agreed. 0. F. Muller M acrobi otus hi/dandi, c. sk, x 

 placed them with the Mites ; 

 Schultze and Ehrenberg near the 

 Crustacea ; Dujardin and Doyere 

 with the Eotifers near the 

 Annelids ; and von Graff with the Myzostomidae and the 

 Pentastomida. Plate regards them as the lowest of all air- 

 breathing Arthropods, but he carefully guards himself against 

 the view that they retain the structure of the original Tracheates 

 from which later forms have been derived. He looks upon 

 Tardigrades as a side twig of the great Tracheate branch, but 

 a twig which arises nearer the base of the branch than any 

 other existing forms. These animals seem certainly to belong 

 to the Arthropod phylum, inasmuch as they are segmented, 

 have feet ending in claws, Malpighian tubules, and an entire 

 absence of cilia. The second and third of these features indicate 

 a relationship with the Tracheate groups ; on the other hand 

 there is an absence of paired sensory appendages, and of 

 mouth -parts. Von Erlanger has pointed out that the Mal- 

 pighian tubules, arising as they do from the mid-gut, are not 

 homologous with the Malpighian tubules of most Tracheates, 



1 Morph. Jahrl. xxii., 1895, p. 491. 



about 350. (From Plate.) a.ep. Epi- 

 dermal thickening round anus ; c/, 

 cloaca; gl.d, accessory gland; <//./, 

 Malpighian gland ; ,<*, stomach ; /*-, 

 testis ; x, mother- cells of spermatozoa. 



