366 PROF. E. R. LANKESTER ON THE MUSCULAR AND 
Tn connexion with this matter it is important to observe that the attachment of all 
muscles in all Arthropods, apparently to cuticular plates or sclerites formed by the 
epidermis externally, is rea//y an attachment to subepidermic connective tissue. No 
muscle ever comes into direct relation with epidermic cuticle, even when that cuticle is 
in the form of a hollow ingrowth (entapophysis) or a solid ingrowth (entosclerites of 
Scorpion). ‘The epidermic cuticle is always clothed internally with fibrous connective 
tissue, and this is the intermediary of the attachment of muscle and sclerite. Accord- 
ingly it is not difficult to conceive of the connective tissue in any special case assuming 
large proportions and dense substance, and if supported otherwise than by its adhesion 
to an epidermic sclerite, losing by degrees all connexion with such a sclerite. 
Applying these considerations to the case of Limulus and Scorpio, we come to the 
conclusion that the muscles attached to the entochondrites are :—lJst, representatives 
of the serial longitudinal intersegmental muscles of the ventral series; 2nd, repre- 
sentatives of the serial dorso-ventral muscles; 3rd, more especially the primitive sterno- 
coxal muscles of the limbs; and 4th, the primitive sterno-buccal muscles. 
In both Limulus and Scorpio the prosomatic entochondrite or plastron, as it is more 
shortly called, represents the midsternal area of several segments fused—probably, in both 
cases, of all the prosomatic segments; though possibly in Scorpio the first segment is 
not included, since muscles to the cheliceree do not arise from the plastron in Scorpio, 
and a longitudinal muscle (84) extends from its anterior subneural processes on each 
side to be inserted into a small postoral sclerite. Probably also the plastron of Scorpio 
includes the midsternal area of the genital (or first mesosomatic segment), since there 
is no separate entochondrite to that segment as there is in Limulus, whilst there is 
such a distinct entochondrite to the next or pectinigerous segment. ‘This view is 
further borne out by the fact that a pair of muscles (the operculo-plastrals 85) similar 
to the muscles (the internal branchials 48) which pass from the entochondrite into each 
of the mesosomatic appendages of Limulus (represented also in the pectinigerous 
segment in Scorpio) pass from the hinder part of the prosomatic plastron of Scorpio to 
the genital operculum. ¥ 
Relative Condition of the Mesosomatic Appendages in Scorpio and Limulus: Lung- 
books and Gill-books.—Vhe six flattened, mesially fused, mesosomatic appendages of 
Limulus are represented by two pairs of appendages and four pairs of respiratory 
lamelligerous cavities in Scorpio. 'The diminutive size of the genital operculum of 
Scorpio as compared with that of Limulus accounts for its incomplete musculature ; but 
such muscles as it has (the operculo-plastrals 85) agree with the more primitive among 
the muscles of the same appendage in Zimulus. A similar statement is true of the 
pectines or second pair of mesosomatic appendages of Scorpio as compared with the 
second pair in Limulus. The four pairs of lung-books of Scorpio are not entirely 
devoid of muscles; the post-stigmatic muscles passing from the posterior edge of the 
stigma of each lung to the posterior border of the sternal region of the same segment 
