ENDOSKELETAL SYSTEMS OF LIMULUS AND SCORPIO. 369 
cating with the cavity of the venous sinus of the animal, as the cavity of the glove does 
with that of the box. 
Now, without removing the glove, push all the fingers from their tips inwards into 
the hand, and then the hand into the box, so as completely to turn the glove outside 
in. ‘Thus the glove will represent the appendage when introverted into the venous 
sinus as in the modern Scorpions. 
The tips of some of the introverted lamellz of the Scorpion’s gill-book have acquired 
laterally, but not in every part, an attachment to the wall of the venous sac into which 
they have pushed their way. ‘These attachments and the relation of blood-space, air- 
space, and cuticle in the lung-lamelle of Scorpio are shown m the transverse sections 
drawn in Pl]. LXXXI. figs. 3 & 4. 
New or non-hereditary Muscles of Limulus. 
The muscles which, if we admit the legitimacy of the hypothesis of de novo formation 
of muscles, must be regarded in the case of Limulus as having come into existence 
subsequently to the divergence of that animal and the Scorpion from a common 
ancestry, and by a process of tissue-change, not by a modification of already existing 
muscle, are the following, viz. the whole series of dorso-ventral muscles which run 
obliquely from the dorsum of one segment to the sternum of another. Such are the 
great dorsal entapophysial-plastral (1), and its branches (83, 34, 85, 86, 87), also the 
ventral entapophysis plastral (2) and its slips (105 to 106); further the dorso-lateral 
plastro-entapophysials (53), the metaplastro-entapophysials (56), the entapophysio- 
metaplastrals (72); the oblique slips (74,75, 76,77); the ventral entapophysiopygals 
(9), and the whole series of branchio-thoracic muscles (18, 19). 
In Scorpio it does not appear that it is necessary to assume a new origin for muscles 
on a similarly large scale. The muscles just noted in Limulus all have relation to the 
peculiar consolidation of the mesosomatic region and the combination of natatory with 
branchial functions in the appendages of that region of the body. In the Scorpion, on 
the other hand, it is the limbs of the prosoma which have become especially developed 
and modified as compared with the archaic plan. In Limulus the limb-muscles of the 
prosoma do not require the hypothesis of any new formations ; they can be derived by 
a process of subdivision from an original hypothetical series of limb-muscles, each limb 
having the muscles which move its coxal segment attached to the adjacent area, 
either of tergum or entochondrite (plastron), which undoubtedly represents the original 
segment to which the particular limb belongs. Not so, however, in the Scorpion, 
where the muscles attached to the coxe of the prosomatic limbs are of great size and 
displaced to such an extent that it cannot be with any confidence asserted that each 
coxa has attached to it merely the modified representatives of the same series of 
muscles which we find repeated in each successive coxa of Iimulus. The disentangle- 
ment of these muscles and their reference to tlie two categories of (a) primary and (4) 
312 
