288 University of California Publications. [Zoology 



and subdivisions will be made until the adult condition is at- 

 tained, when 17 to 20 peripheral spaces may be seen. 



The spaces are sections of longitudinal canals. The endo- 

 dermal cells which separate them in Fig. 32 belong to epithelial 

 ridges extending more or less completely the length of the stem. 

 The height of the ridge cells may vary much, however, even in 

 contiguous sections, as a consequence of which the number of 

 canals visible in different sections of the same stem at this stage 

 may vary irregularly. One large canal may, in the course of 

 a few sections, be divided into two, and at the same time the 

 ridge separating two others may lower abruptly and afford a 

 means of communication between them. Such gaps are usually 

 filled up in the adult, where the peripheral canals are regularly 

 longitudinal, only occasionally branching or laterally in com- 

 munication. 



In general it may be said that the canals appear first near the 

 hydranth, in connection with the fenestrated membrane, decreas- 

 ing in lunnber proximally. 



When the enteric cavity begins to fashion itself into periph- 

 eral canals owing to the intrusion of vacuolated epithelial cells 

 from longitudinal ridges, the intruding cells swell out singly 

 from the epithelial layer. As single cells, also, with their bases 

 on the mural mesogloea, they join in the median line, by means 

 of their inner ends, the corresponding cells from other ridges. 

 Fig. 32 illustrates both of these statements. At this stage, the 

 two essential characters of the adult stem are recognizable : ( 1 ) 

 several canals instead of one, their external walls of low epi- 

 thelium and bounded internally by immense vacuolated cells 

 which separate them from each other; (2) an axis of vacuolated 

 cells occupying the center of the original enteric cavity. 



In the further development of the stem, the canals grow more 

 numerous and the axis of vacuolated cells increases in impor 

 tance, the canals retaining a subordinate peripheral position 

 just within the mesogloea. 



These changes are accompanied (1) by an increase in the 

 number of cells forming the intercanalicular septa (Fig. 33) ; 

 (2) by the growth of a parenchymatous axial tissue out of cells 



