Age -Determination, Growth and Symmetry 27 



AGE-DETERMINATION, GROWTH AND SYMMETRY IN THE 

 TEST OF THE ASCIDIAN, CHELYOSOMA. 



By a. G. Huntsman, B.A., F.R.S.C. 



Comparatively little is known concerning the growth of Ascidians, 

 except during early life. Their embryonic and larval development has 

 been studied repeatedly, as well as the growth of the young individuals 

 during a single season or a year, but so far as we are aware, no attempt 

 has ever been made to determine their length of life or to compare the 

 amou,nt of growth in successive years. Methods have been developed 

 in recent years for determining the age and for studying the growth in 

 the group of the fishes, use being made of the zones or lines of growth 

 that are to be seen in bones, otoliths or scales. The genus Chelyosoma is 

 peculiar among Ascidians in having the test in the neighbourhood of 

 the apertures modified to form definite plates. The latter are outlined 

 by grooves in the outer surface of the test, at the bottom of which the 

 test is quite thin and flexible. Special muscular bands passing from 

 plate to plate are attached to processes of the inner surfaces of the plates, 

 and move them upon each other. These plates show lines of growth 

 which, if our interpretation be correct, make it possible to determine 

 the age and study the growth of an individual. 



We have extended this investigation to show the order and symmetry 

 that obtain in the arrangement and in the mode of formation of these 

 plates in the tbf ee species of Chelyosoma that we have studied. 



Chelyosoma macleayanum, Broderip and Sowerby. 



Our ma,teriai consists of a number of specimens from the Alaskan 

 coast near Point Lay and Icy Cape collected by Mr. F^ Johansen of the 

 Can^adian Arctic Expedition in August, 19 13. In the tw|o smallest speci- 

 mens. Which are quite clear or whitish and 3 and 6.5 mm. in length, the 

 plates are plain and without any indication of interrupted growth in the 

 form of lines. All the larger individuals are yellowish or hojrncoloured, 

 which colour is in the test. In these each plate shows more or less clearly 

 a series of lineis running parallel to the edge of the plate. (PI. i, fig. i). 



Broderip and Sowjerb'y, who gave the first account of the genus and 

 of this species, (i) noticed three or four of these lines in each plate in 

 a specimien from the Arctic regions. In the siphonal plates these lines 

 are parallel to the base or hinge margin and cross each plate from side 

 to side. In the rema^ining plates each line is endless and encloses an 



0) Zoolog. Journ., Vol. 5, p. 46, 1829. 



