194 



CHORDATE ANATOMY 



a wide gulf to be filled between flatworms and annelids. In certain 

 characteristics, the muscles of annelids, it is true, strikingly resemble those 

 of vertebrates. Among these are the segmentation of the muscles, and 

 their separation by a body-cavity into somatic and visceral divisions. 

 It is impossible, however, to be sure that these similarities are not cases 

 of convergence. The eyes of cuttle-fish and of man are similar in many 

 respects, but this does not prove a genetic connexion. 



While the pre-chordate history of muscles is obscure, the evolutionary 

 changes of muscles in chordates are fairly clear. Since the lower chor- 



/ 



MOUTH 



A AMPHIOXUS. 



METAPLEURAL FOLD V LATERAL TRUNK CSOMATIC) MUSCLES 

 GONADS 



QLL APERTURES 

 A 



B. PETROMYZON. 



HYPOBRANCHIAL MUSCLE 



LATERAL TRUNK MUSCLES ANUS 



XJRSAL CONSTRICTORS 

 LEVATOR WAXILLAE f^ _^ 

 SPIRACLE. I ^P^""^ 



pAXIAL TRUNK MUSCLES 



HYPAXIAL MUSCLES 



CLOACAL-ANAL ORIFICE 



Fig. 183. — The lateral trunk muscles of a cephalochordate, a cyclostome, and an 

 elasmobranch, showing their striking metamerism, and fundamental similarity. A, 

 Amphioxus; B, Petromyzon; C, Squalus. 



dates, the Hemichorda and Urochorda, are non-metameric, we must 

 assume that the metamerism of Amphioxus and vertebrates is a new 

 acquisition in the group. The trunk muscles of Amphioxus form an 

 unbroken series of segments extending throughout the entire length of the 

 animal. Each muscle segment or myotome is a mass of muscle tissues 

 which extends around the side of the body nearly to the mid-dorsal 

 and mid-ventral line. Each myotome terminates anteriorly and poste- 

 riorly in connective-tissue septa, the myocommata, which separate suc- 

 cessive myotomes. A sharp bend near the middle of each myotome 

 gives it in side view the shape of a letter V. All alike are innervated by 

 somatic motor nerves. 



