VARIATION IN ASCIDIA. 283 



extremes found in one and the same individual : in these examples the 

 tentacles are in the proportion o£ 4 to 9 in a given space. We are of 

 opinion, however, that the unequal crowding of the tentacles in places is 

 due in part to unequal contraction of the sphincter muscles. Consequently, 

 individuals probably do not differ so much from one another as might be 

 supposed, and authors in describing the condition of the tentacles would do 

 well to take the state of contraction of the sphincters into consideration. 

 The apparent crowding on the dorsal and ventral edges which is sometimes 

 seen is mainly due to muscular contraction. 



Calling the tentacles of the first, second, and third orders 1, 2, and 3, the 

 normal scheme of arrangement in Ascidia ynentula is : — 1, 3, 2, 3, 1 (see 

 fig. 8) ; but there is much individual variation : the third order of tentacles 

 is especially variable, and in places where they are crowded those of the 

 third order are liable to be squeezed out and the remaining tentacles then 

 appear more equal in size. 



The specimens of Ascidia venosa examined ranged from 7 to 11 cm., and 

 the tentacles varied in number from 38 to 74 — the most usual number being 

 from 40 to 50. In this species also there seems to be no correlation between 

 the number of tentacles and the size (probably =age) of the individual. The 

 average proportion of the largest tentacles to the rest is one-half. Here, 

 again, as in A. mentida, there is no correlation between the arrangement of 

 the tentacles and the right and left sides of the body. 



In making a comparison between A. venosa (fig. 9) and A. mentida 

 (figs. 5 to 8), we find that the tentacles in the former are more slender, 

 have a smaller range in number, and show less variation in size and 

 arrangement and spacing, and any crowding noticed seems to be due to 

 muscular contraction. 



Turning now to the Dorsal Tubercle in the same specimens of these two 

 species, we find that it is distinctly smaller in A. venosa than in A. mentida. 

 The typical form in A. mentula is a horseshoe shape with the two free ends, or 

 "horns,'-' turned in the same direction, say to the right side, so that the right- 

 hand horn turns outwards and the left inwards. The commoner variations 

 are that both horns may turn to the left side, or both turn inwards, or one 

 turn inwards and the other remain straight or nearly so. Out of the ten 

 dorsal tubercles examined, six have at least one horn turned to the rio-ht. 



In Ascidia venosa the typical form is a deep U-shape. Out of the ten 

 specimens examined eight are simply U-shaped, and two have one horn turned 

 inwards. 



Finally, we examined the condition of the branchial sac in the same two 

 series of individuals. In comparing branchial sacs it is desirable to examine 

 pieces taken from corresponding parts of the wall ; and, in fact, the samples 

 we have made use of were all cut out from the centre of the right-hand wall 

 about the middle of its length. 



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