SIHI'IIYIAM \ KliTEHI{.\T.\ 1 I o 



Oil I lie s;mu> phitr tlraw and lalx'l a scale showing lines of 

 growth. Also a scale removed from the lateral line showing the 

 pore to the lateral-line ennal. 



Internal Anatomy. Musculature. Keniovc enough of the 

 skin at the base of the tail to expose the thick layer of muscles 

 within. This layer is divided into ziRzag i)ands of muscle tissue. 

 I'iach band is called a muscle segment or myotome. Each myotome 

 extends from middorsal to midventral line and is separated from 

 the adjacent myotomes by connective tissue partitions, calletl 

 mi/osepta. Immcdiatelj' below the lateral line is a longitudinal 

 partition separating the myotomes into dorsal and ventral 

 portions. 



This arrangement of muscles reminds one of tlie state in .\mphi- 

 oxus and is the primitive vertebrate plan from which the muscular 

 system of all land vertebrates is derived. It consists essentially 

 of a longitudinal scries of muscle segments. It is evidence of the 

 segmental plan of structure of the vertebrate body. 



Skeleton. — It will be recalled that in Amphioxus the skeleton 

 consists chiefly of an elastic rod, the notochord; in the shark 

 a more extensive endoskeleton is present and formed entirely of 

 cartilage. In the teleostean fishes cartilage has, for the most part, 

 been replaced by bone. This is the reason for referring to them 

 as the "bony fishes. " During development the vertebral column 

 replaces the notochord, although vestiges of the latter persist. 



Kcfcrcnce to a mounted skeleton reveals the same general 

 divisions that occur in the skeleton of a land vertebrate. These 

 arc an axial skeletoji comprising the skull, vertebral column, and 

 the supports of the unpaired fins and an appendicular skeleton 

 comprising the supports of the paired fiyis. 



Looking further into this internal framework we find the skull 

 divided into three main divisions, the cranium, the sense capsules, 

 and the visceral skeleton. These parts which are separate during 

 development become united in a rather compact whole in the 

 adult. 



The cranium is that portion of the skull which houses the 

 brain. The sensory capsules are skeletal parts which cnchK-^e or 

 support the three pairs of sense organs of the head, the nasal 

 sacs, the eyes, and the internal ears. The visceral skeleton has 

 already been described and studied. 



Note how the bones which support the ilorsal and anal fins 

 alternate with the neural and luemal spines. Note especially the 



