548 
J. S. KINGSLEY. 
extends into the legs. The mesoblastic processes in both 
divide the yolk up into a series of lobes, but these partitions 
never reach clear across the body. A similarity is observable 
in the late appearance of the lumen of the alimentary tract 
cf. Isopods), and apparently in both (certainly in Limulus) the 
hepatic organs are formed from the lobes of yolk which extend 
between the mesoblastic partitions. 
The yolk in Limulus never communicates with the coelom 
as it does in many Hexapods and Crustaceans. Apparently the 
same is true with spiders. 
The heart in Limulus arises by the hollowing out of a solid 
rod of mesoblast, its cells becoming transformed into blood- 
corpuscles. This is paralleled in spiders and in some Crustacea. 
Metschnik off’s account of the origin of the dorsal vessel in the 
scorpions is very improbable and needs confirmation before it 
can be accepted. 
The close resemblances in the segmental organs of the adult 
scorpion and of Limulus have been pointed out by Lankester 
(’82 and ’ 84 ), though in his later paper he does not refer to 
their possible homology with the segmental organs of worms, 
(suggested by Packard ’ 75 a ) as he was unable to find any 
ducts. As described above I have found these ducts in the 
young, and doubtless the same result will be obtained when 
the young of the scorpion is studied by means of sections. 
This is the more probable since Bertkau (’ 84 ) has found homo- 
logous glands in various spiders, which were without external 
openings in the adult, but in the young of Atypus the duct 
was found to open at the base of the third pair of legs (fifth 
pair of appendages.) 1 Michael (’ 83 , p. 21) describes similar 
glands in the Oribatidae but failed to find their openings. 
The exact correspondence of these glands both in position and 
in their ducts, and their closely similar histological structure, is 
a point of no little importance in the argument for the union 
' These glands have long been known in spiders, but had been regarded 
generally as belonging to the digestive tract. They were practically discovered 
and their true structure first described both in Scorpions and Spiders by 
Professor Lankester. 
