The Pelagic Tunicata of the Gulf Stream. 87 



THE MUSCLES OF THE AGGREGATED CYCLOSALPA. 

 (Plate I, figs. 5 and 6; plate 2, figs. 8 and 9.) 



In the aggregated forms of the various species of Cyclosalpa, the muscles 

 at the anterior end of the body, which I have numbered from i to 8, are so 

 much alike that they can be identified without difficulty, both with each 

 other and with the muscles of the solitary Cyclosalpa. There is much greater 

 specialization in the muscles in the posterior region of the body, which I 

 have numbered from 7 to 12. Here it is difficult to trace exact homologies 

 among the aggregated forms of the various species, or between the aggre- 

 gated form and the solitary form, although it is probable that the muscles 

 of all the cyclosalpas, solitary and aggregated, are homologous. 



In all the aggregated forms there is a muscle that is very characteristic. 

 It is marked C in figure 5, S. floridana, and is shown unlettered in text-figure 

 2, 6". pinnata, and text-figure 3, 5\ affinis. It extends obliquely upwards and 

 backwards from one of the encircling muscles to the next. As it appears to 

 be equivalent in all the species, I assume that the muscles that it joins are 

 also equivalent, and I have designated them by the same numbers, 9 and 10. 

 In all the aggregated salpas the stalks that bind the members into a com- 

 munity contain prolongations of the body-cavity into which two or more of 

 the muscles are prolonged, as is shown in text-figures 2 and 3, and these 

 appear to be homologous in the different species. 



The muscles of the aggregated 5\ floridana are shown in plate i, figures 

 5 and 6, and plate 2, figure 9; those of the aggregated 5\ pinnata in plate 2, 

 figure 8, and in text-figure 2 ; those of 5\ affinis in text-figure 3. 



Muscle i crosses the middle line of the dorsal surface, just posterior 

 to the slender oral muscles, and, bending around on to the ventral surface, 

 runs backwards, and, uniting with muscle 5, forms muscle 6, resembling 

 the corresponding muscle of the solitary form in all respects. 



Muscle 2 is a longitudinal muscle which connects muscle i with muscle 



4, as in the solitary form. 



Muscle 3 is prolonged ventrally into the attaching process, and, running 

 dorsally, posterior to the anterior end of the endostyle, unites with muscle 

 2 to form muscle 4, as in the solitary form. 



Muscle 4 is a dorsal longitudinal muscle that is formed by the union 

 of muscle 2 and muscle 3, as in the solitary form, and it unites with muscle 



5, as in the solitary S. floridana, and gives rise to a muscle, A, which crosses 

 the middle line to unite with its fellow. 



Muscle 5 unites with muscle i to form muscle 6, as in the solitary form, 

 but it unites dorsally with muscle 4 to form muscle A. 



Muscle 6 is formed by the union of muscle i with muscle 5, as in the 

 solitary form. In S. affinis and S. pinnata it unites ventrally with muscle 

 7 and prolonged into the attaching process, where it unites with muscle 

 7 to form muscle E. In 5\ floridana it crosses, but does not unite with, 



