MUSEUM OF COMPARATIVE ZOOLOGY. 19 



anal end than in the last stage, and there is a second cavity below the 

 atrium, from which it is separated by a line of nuclei. This is plainly 

 an early stage in the formation of the alimentary tract, which thus first 

 appears at the anal side of the bud, as in Phylactolsemata, and is progres- 

 sively formed towards the oral end. An appearance similar to the one 

 figured would be given by a slightly oblique section of a later stage; but 

 this section is strictly sagittal, and no trace of the lumen appears in adja- 

 cent sections. I have found a similar condition in a series of longitu- 

 dinal sections at right angles to the sagittal plane of the bud (Plate IV. 

 Figs. 39 and 40). Figure 39 shows that the atrio-pharyngeal cavity is 

 first developed at the anal end, and in Figure 40, which is three sections 

 (about 15 /A ) below Figure 39, the anal end only of the alimentary tract 

 is formed. It is worthy of notice that the cells of the mesodermic layer 

 of the bud are often greatly vacuolated at this stage, as in Figures 39 and 

 40, vac. Braem ('90, p. 126) says of this stage : "Die der Resorption 

 dienenden Darmabschnitte, Magen und Enddarm, werden gemeinsam an- 

 gelegt, indem auf jeder Seite der Kuospe eine Langsfalte die Wandungen 

 nach innen und gegen einander zu einbiegt, worauf die benachbarten 

 Theile des inneren Blattes verschmelzen und so durch eine Art Abschniir- 

 ung das primiire Knospenlumen in den vorderen Atrialraum und die 

 hintere Darmhohle getrennt wird." While I thoroughly agree with this 

 statement, the additional fact of the formation of the tract progressing 

 from the anal towards the oral end is interesting, in that it shows that the 

 process of formation of the organ in Paludicella is fundamentally similar 

 to, although differing slightly in detail from, that of the Phylactolsemata. 

 Figure 24 shows in sagittal section a still later stage in the development 

 of the alimentary tract. A cross section of this stage is seen in Figure 

 30 (Plate IV.), in which the separation of atrial and gastric cavities is 

 demonstrated. The inner layer of the bud is here seen to be separated 

 from the ectoderm by a distinct line, and, to a certain extent, even by 

 the mesoderm. The distal (oral) part of the cavity of the alimentary 

 tract next becomes considerably enlarged to form the stomach (Fig. 25). 

 The outer layer of the bud, ms'drin., penetrates between the stomach 

 and the atrium, and a depression is formed at the bottom of the atrial 

 chamber which will give rise to the oesophagus. Even at this stage the 

 oesophagus is not in communication with the stomach, but their cavities 

 are separated by two layers of cells of the inner layer of the bud. These 

 two layers become those of the cardiac valve (Plate IV. Fig. 36, vlv. cr.). 

 By a further comparison of Figures 25 and 36 it will be noticed that, 

 whereas in the earlier stage, as in Endoprocta, there is no coecuui to the 



