366 PROF. E. E. LANKESTER OX THE MUSCULAR AND 



Til 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 really 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 iutemally 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 : — 1st, 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 Lbmilus and Scorpio the prosomatic entochondrite or plastron, as it is more 

 shortly called, represents the midstcrnal area of several segments fused — probably, in both 

 cases, of all the prosomatic segments ; though possibly in Scorpno the first segment is 

 not included, since muscles to the chelicerse do not arise from the plastron in Scorpio, 

 and a longitudinal muscle (84) extends from its anterior subncural 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. — The 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 oi 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 Limulus. 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 Scorpno 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 



