262 



COMPARATIVE ANATOMY 



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. Moreover, there 

 are striking dissimilarities between annelids and vertebrates, such as the 

 circular trunk muscles of annelids which have no homologues in verte- 

 brates, and which consequently make it difficult to accept the hypothesis 

 of the annelid ancestry of vertebrates. 



METAPLEURAL FOLD \/ LATERAL TRUNK CSOMATIC) MUSCLES 



A AMPHIOXUS. 



OLL APERTURES 



BPETROMYZON. 



HYPOBRANCHIAL MUSCLE 



LATERAL TRUNK MUSCLES ANUS 



^RSAL CONSTRICTORS 

 SPIRACLE 



pAXIAL TRUNK MUSCLES 



MOUTH 

 C.SQUALUS 



HYPAXIAL MUSCLES 



Fig. 217. — 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. 



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

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

 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 fibers 

 which extends around the sides 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 



