358 WILLIAM PATTEN. 



appear completely with the exception of a few longitudinal 

 muscles passing from the occipital region of the cranium to the 

 pectoral (= pectinal) arch. 



It is important to observe that in Arthropods, according as 

 the anterior portion of the body becomes more specialised, the 

 appearance of segmentation in those regions is retarded. For 

 example, in insects, segmentation appears in the cephalic lobes 

 and in the region of the mandibles and first maxillse later than 

 in the rest of the body ; being the exact reverse of what we 

 should expect, since the anterior part of the body is the oldest. 

 It is the same with the cephalic lobes of Scorpio. In 

 Limulus the fourth, fifth, and sixth (?) thoracic segments 

 appear first ; the third, second, and first later, and those of the 

 fore-brain last of all. 



In view of these facts it is probable that in Vertebrates what 

 has been taken for an intercalation of new head-segments is 

 really a retarded segmentation. 



Eye Muscles. — In Scorpio, and probably in most Arach- 

 nids, there is a small number of muscles which belong neither 

 to the system of leg muscles nor to the longitudinal ones. 

 They are dorso-ventral, and in Scorpio the largest one is 

 attached to the hfemal wall of the head on either side of, and 

 close to, the median eyes. Although I have not been able to 

 follow the development of these muscles, it is almost certain 

 they develop from the first two or three thoracic somites. No 

 such muscles are found in the posterior part of the thorax. 

 They thus agree to some extent with the Vertebrate eye 

 muscles, for the latter arise, as Van Wyhe has shown, inde- 

 pendently of the eye from the first three somites, and belong 

 neither to the primitive gill-arches nor to the longitudinal 

 muscles. 



XII. Pectoral Fins. — There are four completely fused 

 segments in the vagus region of Scorpions, the comb belonging 

 to the third. As there is no external evidence of this con- 

 dition in the adult, we must, in attempting to determine the 

 homologies of the appendages in such forms as Merostomata, 



