10 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



all the muscle-bands as interrupted ventrally; so hard is it to correct 

 established error. 



The atrial aperture is sometimes valvular, as, for example, in the 

 aggregated form of Salpa cordiformis, Plate III, Fig. 2, g v , but it is usually 

 a simple opening, circular or nearly so, without valves. Its mode of origin 

 in the embryo of Salpa pinnata is shown in Plate XVII, Figs. G and 7. 

 Quite frequently it is produced into a projecting funnel, as in Salpa costata, 

 Plate IV, Fig. 4, g v . It may be terminal at the posterior end of the 

 body, as in Salpa pinnata, Plate I, Figs. 1, 3, 5 and 6, or it may be on the 

 upper surface of the body at some distance from the posterior end, as in 

 Salpa democratica, Plate II and Plate XLIII, Fig. 1. There is little 

 uniformity in its position. In Salpa democratica it is on the upper sur- 

 face in both the solitary form and the aggregated form. In Salpa pin- 

 nata it is terminal in both forms, as it is also in Salpa costata. In the 

 solitary form of Salpa cylindrica, Plate III, Fig. 5, it is terminal, while 

 it is on the upper surface of the aggregated form, Fig. 6, while the reverse 

 of this is the case in Salpa hexagona, Figs. 4 and 1. 



The atrial aperture is usually encircled by sphincter muscles, as in 

 Salpa cylindrica, Plate III, Fig. 5. These muscles are usually a continua- 

 tion of the series of body-muscles, which gradually become narrow and 

 crowded together as they approach the cloacal aperture. In these cases 

 the atrial sphincters are often complete rings. In a few species they 

 have a more complicated arrangement, as in Salpa costata, Plate IV, 

 Fig. 4 and Plate VIII, Fig. 4, where they branch out in a dendritic man- 

 ner from two lateral longitudinal trunks. The chain-form of Salpa cor- 

 diformis, Plate IV, Fig. 6, has the same arrangement in a rudimentary 

 or slightly developed condition. Often there is a lack of bilateral sym- 

 metry in the posterior body-muscles of aggregated salpae. In the aggre- 

 gated form of Salpa costata, Plate VIII, Fig. 4, the branches of one lateral 

 trunk are very slightly developed, while those from the other almost 

 encircle the atrial funnel. 



The last body-muscle of the aggregated form of Salpa cordiformis, 

 Plate III, Fig. 2, is forked on the right side but not on the left. 



As Leuckart pointed out long ago (I, p. 6), the mouth and the atrial 

 aperture are very much nearer each other in the young salpa than they 

 are in the adult, as examination of my figures will show. 



Figure 2 of Plate XLI shows a young embryo of Salpa pinnata with 

 the point where the mouth is to be formed marked r, and the position of 

 the atrial aperture marked g v . It will be seen that the interval between 



