ENDOSKELETAL SYSTEMS OF LIMULUS AND SCOEPIO. 369 



eating 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 lamellae 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-lamellse of Scorpio are shown in the transverse sections 

 drawn in PI. 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, 84, 85, 86, 87), also the 

 ventral entapophysis plastral (2) and its slips (103 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 coxa? 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 Limulus. The disentangle- 

 ment of these muscles and their reference to the two categories of (a) primary and (b) 



3i2 



