ANATOMY OF AMPHIOXUS. 



This peculiar plate-like muscular tissue is found in 

 connection with the lateral muscles only of the Cyclostome 

 fishes. The muscle-fibres of the mouth 

 and velum, as LANGERHANS pointed out, 

 closely resemble those found in the walls 

 of the heart of the higher Vertebrates. 

 In transverse section the cut edges of 

 the longitudinal muscle-plates are to be 

 seen stretching across the myotome. 

 (Cf. Figs. 2, 26.) 



The transverse or sub-atrial muscles are 

 divided by a median longitudinal septum 

 of connected tissue into right and left 

 halves. They are further subdivided into 

 a series of compartments by thin trans- 

 verse septa. These compartments, how- 

 ever, are not arranged segmentally, since 



Fig. 16. Isolated 



muscle-fibre of the they are more numerous than the myo- 

 tomes. The muscle-plates of these mus- 



cles are placed edge on, so that they do 

 not lie one over the other as the plates of the myotomes 

 do, but one behind the other. They are attached to the 

 septum at the base of the myotomes on the one hand, and 

 to the median septum or raplic on the other, and also they 

 are attached at numerous points to the connective-tissue 

 sheath or fascia which covers them above and below. 

 When they contract, therefore, the floor of the atrium is 

 thrown into a number of characteristic pleats. (Cf. 

 Fig. 2.) The individual muscle-plates of Amphioxus ap- 

 pear universally to be devoid of a protecting sheath or 

 sarcolemma. The sub-atrial muscles end at the atriopore, 

 round which they form a sphincter muscle. 



The muscles of the oral hood, which serve for the erec- 



