322 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



published the only indication of variation is in Swederus' 

 description, where the number of plates is stated to be 

 nine, instead of eight. Five figures of the disk have been 

 printed and in all the plates are the same. Besides these 

 there is one figure of Wagner's which would seem to indi- 

 cate that the two anterior plates are wanting in that 

 individual; the appearance, however, may be merely due 

 to the perspective. The reason for this constancy lies in 

 the fact that the muscles extend across all the hinges and 

 are perpendicular to them. This is the most stable condition, 

 and deviations of either muscles or hinge would cause no 

 rearrangement, but would tend to be gradually obliterated 

 by the interaction of the two. In accordance with this 

 view we find that in C. -productum, which is on the whole 

 very variable, the siphons, in which the principal muscles 

 cross the hinges at right angles, are remarkably constant. 

 But on the other hand, the peripheral muscles are nearly 

 parallel to the hinges of that region, and here we have the 

 greatest variability. The conditions are unstable, for a 

 slight deviation in the disposition of the muscles can easily 

 cause a rearrangement. Thus, if in the large lateral plate 

 A fig- 3> a slight massing of the muscles at the two ends 

 might have resulted in the formation of two plates there, 

 just as has occurred on the opposite side. Similarly, the 

 presence of one or more longitudinal hinges between the 

 central plates, which is the most constant difference in the 

 two disks, follows as a mechanical necessity from the 

 arrangement of the muscles. Thus it becomes evident that 

 all the differences in the disks of the two forms are cor- 

 related with the great development of the peripheral 

 muscles in C. productum. 



The Pharyngeal Apparatus. 



The simple filiform tentacles are subject to considerable 

 variation, which is independent of the size of the individual 

 as the following measurements show: 



