8 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



shows that the upper lip is rounded and protuberant and thick, while 

 the lower lip is very thin. In mature specimens this thin lower lip folds 

 inwards so that the thickened upper lip shuts closely upon it when the 

 mouth is closed. The flexible edge of the lower lip acts as a valve, for it 

 is depressed by the inflowing current of water, while back pressure folds 

 it up against the upper lip and closes the mouth. In many species there 

 are two short muscles on the upper surface of the upper lip to open it. 

 They lie near the middle line, and they are parallel or nearly parallel to 

 each other. They are shown in Plate II. 



The muscles for closing the mouth are much more complicated, as 

 many of the figures show : Plate IV, Fig. 2, for example. Their arrange- 

 ment in the solitary salpa is usually different from that in the aggre- 

 gated form, and they also vary according to the species, but in most 

 cases some approximation to the arrangement shown in Plate XLI, Figs. 

 3 and 5, can be made out, although the oral musculature is seldom as 

 well developed as it is in these figures, which represent solitary embryos 

 of Salpa pinnata. The mouth is not yet open in these embryos, but r 

 marks its position. A bridle-like muscle arises from about the middle 

 of the second body-muscle, the body-muscle which crosses the upper 

 surface of the body just behind the ganglion, s, and runs across the 

 oral end on what is to become the angle of the lower lip, and, crossing 

 the middle line, unites to the middle of the second body-muscle on the 

 opposite side. At its lower end this body-muscle joins a muscle which 

 runs upwards and forwards to the angle of the mouth, where it divides 

 into three branches, two superior ones which cross the middle line on the 

 convex surface of the upper lip, and one inferior one which crosses the 

 middle line just below the mouth. These muscles are shown in section 

 in Plate XLV, Fig. 8. In adult salpa3 the oral muscles are usually quite 

 distinct from the body muscles in their anatomical relations, and very 

 much smaller; but these embryos show that they belong to the system 

 of body-muscles, or perhaps it is more near the truth to say that the 

 body-muscles of salpa are modified oral and atrial sphincters. 



The body-muscles exhibit the greatest diversity of arrangement, as 

 may be seen by comparing the more divergent forms, such as Salpa 

 hexagona, Plate III, Fig. 4 ; Salpa scutigera, Plate IV, Fig. 7, and Salpa 

 costata. Fig. 4. 



It is hard to say what number of muscle-bands is most characteristic ; 

 nine seems to be more common than any other number, but in some 

 species, Salpa scutigera for example, they are reduced to four, while in 



