THE MUSCLE BANDS OF SALPA. 55 



Fig. 11. Normal form of Operculum ; Figs. 12 14. Abnormal 



forms of same. 

 Fig. 15. Operculum of 8. reversa. Note : All the figures are 



drawn to the same scale. 



III. ABNORMALITIES IN THE MUSCULAR BANDS OF 



Of over 200 specimens of larval Salpa mucronata-demoeratica 

 (solitary form) that I recently examined one by one, only some three 

 shown any marked sign of abnormality. Of these, two are figured 

 PI. v, Figs. A, B, and C. The first two figures, A and B, represent the 

 two lateral views of the same animal ; C, the dorsal view of another. 



In every respect saving in that of muscle arrangement these 

 individuals were quite normal, but C showed a blending of bands iii 

 and iv for a short distance, as shown, and this case is of comparatively 

 little importance. The other example, figured in C and D, is a 

 much more curious instance, and is of high value to the student of 

 variation. Therein all the muscle bands save the 1st and the 7th, 

 branch and anastomose in an extremely complicated manner ren- 

 dered clear however, by reference to the drawings. 



Without entangling ourselves in the rival theories as to the 

 primitive character or otherwise of the pelagic Tunicates, the 

 variation I now record can be used as one link in the chain of true 

 evidence that will some day be forged explanatory of Ascidian 

 descent. Probably the variation is atavistic, and if so, points directly 

 to a state when the musculature was not a nearly regular arrange- 

 ment of more or less encircling bands, but was continuously, or at 

 least irregularly, disposed over the whole body, perhaps much in 

 the manner now 7 to be seen in the muscle arrangement of- Ascidia 

 mentula. 



Gegenbauer (Comp. Anatomy, 1878, p. 395) from reasoning 

 based on other foundations, arrived at the same result, stating that 

 in the Thaliacea, " the hoop-like formation arises from the differen- 

 tiation of a primitively continuous muscular layer. Gaps arising in 

 this, became gradually larger until the breaking up of the layer into 

 separate hoops is brought about " ; a statement that receives very 

 important direct confirmation from the instances of irregular banding 

 that I have figured. 



I may add that the abnormal specimens, described in the 

 above note, as well as in the preceding one dealing with the Serpulid 

 operculum, are carefully preserved in the permanent collection of the 

 Jersey Marine Zoological Station. 



