MUSEUM OF COMPARATIVE ZOOLOGY. 131 



approximately the same. The free ends of the arms are shown in 

 Figure 99, Plate XL, just above loph'. The polypide figured here is 

 only slightly older than that of Figure 77, Plate IX. The connection 

 between the two arms is not one of contact merely, for in the region 

 of fusion one can count roughly three layers of nuclei, whereas each of 

 the two free portions of the same cell layer contains but one layer 

 of nuclei (Fig. 99). 



Before the atrial opening is formed, a separation of the two arms 

 begins to take place. This process commences at the base of the arms, 

 and proceeds upward as the tentacles of the inner row successively 

 reach a certain stage of development. As the work of separation pro- 

 gresses, the cells of the connecting band lose their capacity for becoming 

 stained and appear vacuolated. The vacuoles increase in size until the 

 connection between the arms is reduced to a series of fine threads 

 (Plate VIII. Fig. 75, lophJ), which arc probably sundered when the ten- 

 tacles of the inner row (can. crc", Fig. 76) bend at right angles to their 

 former position to become parallel to those of the outer row. In at- 

 tempting to find an explanation of this process, it must first be ascer- 

 tained how the arms of the lophophore grow in length. One is perhaps 

 inclined to think of a terminal growth, but this does not take place. 

 So far as I can judge from an examination of many longitudinal sec- 

 tions of the arms, cell proliferation goes on throughout the whole length 

 of the arm, and with nearly equal rapidity in all parts. The distance 

 between the centres of the terminal tentacles is about the same as in 

 the case of the more fully developed proximal ones, but they are closer 

 together in the young arm than in the adult one. This being the case, 

 there ought to be as many (incipient) tentacles in the young as in the 

 adult, and I find that to be, so far as I can determine, very nearly or 

 exactly the case. 



The horseshoe-shaped lophophore being characteristic of the Phy- 

 lactolsemata, a study of its development is important, since it may be 

 expected to throw light on the phylogeny of the group. We have in 

 Cristatella, Plumatella, and Fredericella, a series in Avhich the arms of 

 the lophophore are shorter and shorter, in correspondence with other 

 changes, by which is effected a gradual transition to the Gymnolaemata, 

 which have a circular lophophore. In Gymnolaemata, the ring canal 

 lies at the base of all tentacles in the adult. The anus lies outside the 

 circle of this canal. The brain lies within the lumen of the canal. 



Nitsche ('71, pp. 43-45) has given the best description extant of the 



