132 bulletin: museum of comparative zoology. 



manent ectoderm. The ectodermic buds in their further growth continue 

 to carry the connective-tissue mesoderm before them until they come into 

 contact with the wall of the digestive tube. The connective-tissue par- 

 tition between the successive masses of ectodermic cells may be clearly 

 seen in longitudinal sections of stages as advanced as that of Figure 16 

 (Plate 3), the anterior masses being more clearly separated than the 

 posterior ones. 



The blood-vessels undergo considerable branching and anastomosis in 

 the anterior and posterior ends of the mature animal. In the part of 

 the adult worm where budding is to take place, the dorsal and ventral 

 vessels are not connected by conspicuous circumintestinal loops ; but in 

 the budding zone, long before separation of the zooids, a complete vas- 

 cular loop is formed in each zooid close to the plane of division (Plate 3, 

 Fig. 17, vas. sng.crc). The complex anastomosis is not, so far as I can 

 determine, completed until the animals become independent. The loops 

 are of very small calibre until this occurs. 



d. Summary of Changes in the Formation of the Mouth and 



Pharynx. 



In concluding this part of the subject, it is perhaps desirable to sum- 

 marize under a separate heading the chief features connected with the 

 rather complex processes involved in the formation of the structures 

 appropriate to the anterior end of the digestive tract. This is ren- 

 dered desirable from the fact that each of the germ layers contributes 

 to its production. 



The wall of the new pharynx is formed around the old gut by a 

 proliferation of the cells occupying the base of the original epithelium, 

 and a delamination of the layer thus formed. The lumen of the pharynx 

 exists, as a lacuna dorsal to the gut, between the old and new entoderm 

 before the separation of the zooids. Dorsally, the new entodermal wall 

 does not reach forward beyond the position of the brain. Laterally, its 

 anterior margin passes downward and somewhat backward, but turns for- 

 ward again in the middle of the ventral side of the worm. To the curved 

 lateral margin of this new wall is applied a pair of ectodermal ingrowths, 

 whose history is as follows. 



The groove marking the region of separation of the zooids, relatively 

 shallow in other regions, becomes specially deepened at two points, sym- 

 metrical to the median plane, and corresponding to the space between the 

 ventral and lateral longitudinal muscles. The axis of these depressions 

 lies in the plane of the groove, which inclines forward, and dorsally. The 



