86 Papers from the Marine Biological Laboratory at Tortugas. 



Muscle 9 is nearly transverse dorsally, whereas its ventral half runs 

 inwards and forwards. 



Muscle 10 is like muscle 9, except that its ventral half is still more in- 

 clined forwards. 



Muscle n is like muscle 9 and muscle 10, except that its ventral half is 

 still more inclined forwards. 



Muscle 12 crosses the middle line of the ventral surface, while its dorsal 

 portions do not meet on the middle line. 



Muscles n, 13, 14, and 15: I am not able to trace an exact homology 

 between the muscles posterior to muscle 12 in the various species of Cyclo- 

 salpa, although this failure may be due to a lack of detail in this region in 

 the figures. 



Muscle 1 6, which occurs also in S. pinnata, is shown in 5". floridana in 

 figure 7, is a longitudinal muscle that connects muscle I with muscle 5. It 

 is not visible in a symmetrical dorsal or ventral view, as it is on the side of 

 the body, midway between the dorsal and the ventral surface. It is shown 

 in the adult 5*. floridana on the left side of figure I, which is not a perfectly 

 dorsal view. 



The four known species of Cyclosalpa fall into two sets as regards the 

 muscles of the solitary form. In Salpa pinnata and 5". affinis the muscles 

 are not united into groups or bundles, while they are so united in 5". virgula, 

 and S. floridana. 



In S. floridana (plate i, figure i) muscles 4, 5, and 7 of each side are 

 united near the middle line of the dorsal surface into a single muscle, which 

 does not cross the middle line nor unite with its fellow of the opposite side. 

 In the ventral surface of the same species (figure 2) muscles 6, 7, 8, 9, and 10 

 unite to form a common trunk, which crosses the middle line of the ventral 

 surface to become continuous with its fellow of the opposite side. 



Since the muscles of the solitary cyclosalpas are homologous, and those 

 of 6". pinnata and 5*. affinis simple, or not united into groups, while those 

 of S. floridana and S\ virgula are more specialized, it seems natural to re- 

 gard 5\ pinnata and S. affinis as more primitive. The study of the digestive 

 organs corroborates this view and seems to indicate that all the cyclosalpas 

 are derived from a pinnata-like ancestral form. 



In the solitary S. pinnata muscles 7, 8, 9, 10, and n are incomplete both 

 dorsally and ventrally, and they are not joined into bundles. In S. affinis 

 muscles 8, 9, 10, and n are complete dorsally, but not united into bundles. 

 In S. floridana they are incomplete dorsally, but 4, 5, and 7 are united into 

 a bundle on the dorsal surface, while muscles 6, 7, 8, 9, and 10 are com- 

 plete ventrally and united into a bundle. 



