490 MAYOR. 



which the conclusions of the paper are based. Although only indi- 

 vidual differences have been observed between the opposite sides of 

 the bilaterally symmetrical planulae, it has been thought best for 

 convenience in description to distinguish a right and a left side. 

 In this connection the aboral end of the planula, which is foremost in 

 locomotion, has been considered anterior while the oral end is con- 

 sidered posterior. 



Each larva was cut into a series of transverse sections of equal thick- 

 ness, which varied from 5 /x to 7 ^i for the different series. In record- 

 ing the position of structures in the larvae the thickness of the sections 

 has been used as a unit of measure. 



The nine larvae fall into four groups: — First, larva A, which has 

 four pairs of mesenteries, only two of which are well developed ; Second, 

 larvae B, C, D, and E, in which there are six pairs of mesenteries, the 

 fifth and sixth however being only slightly developed and the mesen- 

 terial filaments not extending the whole length of the larva; Third, 

 larvae F, G and H, in which all six mesenteries are well developed 

 and the mesenterial filaments of the first two pairs of mesenteries 

 extend through the greater part of the larvae; Fourth, larva I, in 

 which, besides the greater development of the fifth and sixth pairs 

 of mesenteries, there are developed mesenterial filaments on the 

 third pair of mesenteries. 



The musculature of the mesenteries consists of fibres developed in 

 endoderm cells where these abut on the mesogloea. These fibres 

 stain deeply with haematoxylin and so are easily distinguished in the 

 preparations. In the mesenteries the majority of these fibres run 

 longitudinally. Some, however, especially those near the junction 

 of the mesogloea of the mesentery with that of the body wall, run 

 obliquely or even transversely. It is not always easy to deter- 

 mine which side of the mesentery shows the large number of fibres; 

 first, because the mesogloea in the mesenteries is usually very thin 

 and does not take stain well, and, secondly, because there are always 

 some fibres on each side. This is still more difficult in the newly 

 formed mesenteries, as in these there is almost always an approxi- 

 mately equal number of fibres on each side of the mesentery. The 

 distribution of the muscle fibres given in the diagram (text Fig. 1, C) 

 is based on careful examination with a 2-mm. apochromatic objec- 

 tive and compensating oculars x8 and xl2. In the first and second 

 pairs of mesenteries the muscle fibres are confined almost entirely to 

 the ventral sides. In the third and fourth pairs, the directives, the 

 muscle fibres are more numerous on the lateral sides. In the fifth 



