116 bulletin: museum of compaeative zoology. 



Cambridge. At this time it usually occurs as single individuals or with 

 only two zooids in a chain. These, collected and placed in a suitable 

 aquarium, will thrive and multiply asexually throughout the winter. 

 The rate of division depends upon the food supply, by the regulation 

 of which certain interesting variants from the normal may be had. 



2. Structure of the non-dividing Animal and the Formation 



of Segments. 



In individuals not sexually active and not in process of budding, the 

 following regions may be distinguished: (1) prostomium ; (2) four 

 cephalic segments bearing ventral bristles only ; (3) twenty to twenty- 

 five well developed body segments, with both dorsal and ventral bristle 

 bundles ; (4) a region of incompletely formed segments passing pos- 

 teriorly into an undifferentiated region or zone, — all of which may 

 equal tw^o or three adult body segments in length ; and (5) a completely 

 developed, complex structure, — representing probably at least one seg- 

 ment (anal segment), — which bears the anal orifice, the respiratory 

 lobes (pavilion), and the digitiform appendages. 



The formation of new segments in such a worm is a process which 

 presents so many points of similarity to that of budding, that I wish to 

 give a brief description of it before proceeding to the consideration of the 

 latter phenomenon. New segments are invariably formed immediately 

 in front of the anal segment, and always from the anterior portion of the 

 undifferentiated zone. The length of this preanal zone is greater in well 

 nourished than in poorly nourished individuals. In this part of the body 

 all the structures which characterize segments in the more mature regions 

 are less and less difierentiated as one proceeds posteriorly. Even the 

 organs which pass through this region in a functionally complete con- 

 dition, such as the intestine, the blood-vessels, and, in part, the nerve 

 cord, present simpler conditions than they do farther forward. The 

 nerve chain, for example, is represented chiefly by non-fibrous elements, 

 only a few fibres passing through to innervate the pavilion. The ventral 

 and lateral portions of the body cavity are shown, by transverse sections, 

 to be filled with a mass of indifferent tissue derived from cell multiplica- 

 tions in the ectoderm, the products of which break through the muscular 

 layers at definite places and then fuse together, much as in the budding 

 process (Plate 2, Figs. 8, 9, Plate 3, Fig. 15) described later. These 

 ectodermal ingrowths are arranged serially in the long axis of the worm, 

 and afford the earliest signs of segmentation. 



