xIX DEVELOPMENT—AFFINITIES 483 
The segmentation of the egg in JZ macronyx is total and 
equal, according to the observations of von Erlanger.' 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 
: xe Fig. 254.—Male reproductive organs of 
means agreed. OL EF. Muller Macrobiotus hufelandi, C. Sch., x 
placed them with the Mites ; about 350, (From Plate. ) a.ep, Epi- 
‘ dermal thickening round anus; ¢/, 
Schultze and Ehrenberg near the cloaca; gi.d, accessory gland; gi./, 
Crustacea ; Dujardin and Doyeére We Sue gland ; a SuOnE te, 
is f testis ; 2, mother-cells of spermatozoa. 
with the Rotifers near the 
Annelids; and yon 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. Jahrb. xxii., 1895, p. 491. 
