STRUCTURE OF THE AMAROUCIUM TADPOLE 87 



TAIL MUSCLES 



Two muscle bands, each consisting of about eighty very large, 

 polygonal muscle cells arranged in four longitudinal rows of 

 twenty cells each, form the dorsal and ventral portions of a 

 relatively thick envelope for the notocord (figs. 2 and 8). A 

 single layer of cross-striated contractile fibrillae are differen- 

 tiated in the cortical layer of each muscle cell. The fibril- 

 lae take a general longitudinal course, but are inclined about 

 18° to the right of the longitudinal axis of the tail. The fibril- 

 lae of adjacent muscle cells join end to end and thus convert the 

 entire series of muscle cells of each muscle band into a single 

 muscle. A further indication that the muscle band, rather than 

 the individual muscle cell, is the morphological as well as the 

 physiological unit, is afforded by the fact that the alternate light 

 and dark segments of the fibrillae are so placed that they form 

 continuous straight transverse rows or lines across the muscle 

 bands, which are not in any way interrupted or interfered with 

 by the muscle cell walls. Each muscle band functions as a 

 unit in a way that indicates that its origin is located at the an- 

 terior end of the notocord, its insertion at the posterior end. 

 With each muscular contraction the tail makes a propeller-blade- 

 like stroke, due to the oblique or spiral course of the contractile 

 fibrillae in the muscle bands, and the body of the tadpole is thus 

 made to rotate clockwise during locomotion. 



The central portion of each muscle cell contains a nucleus 

 and cytoplasm in which vacuoles, pigment granules, and larger 

 spherical inclusions are usually found (figs. 2 and 8). 



ALIMENTARY TRACT 



The pharynx, which at this stage is not organically connected 

 with the oral siphon, occupies a large portion of the median 

 dorsal part of the body (fig. 1). In the part of its dorsal wall 

 situated immediately in front of the oral siphon, the endostyle 

 is conspicuously differentiated as a double longitudinal fold. 

 A deep groove is included between the folds, at the bottom of 

 which a tract of long cilia is developed (figs. 1 and 2) . The ventral 

 wall of the pharynx is intimately associated, especially at its 



JOUHNAL OF MOKPHOLOQY, VOL. 36, NO. 1 



