TUBULARIA SOLITARIA. 87 



the basal expansion of the hydranth (figs. 5 and 12, h. t.). 

 The endoderin of the tentacles is continuous with the middle 

 layer of the endodenn of the polyp (r.ed.). It is far from 

 reg'ularly septate in character, although it approaches this 

 condition more closely than in the case of the distal tentacles. 



The mesoglea projects inwards between the hydrocaulus 

 and the hydranth, forming a kind of diaphragm, and leaves 

 open only a small pore of communication (fig. 5, d.). The 

 possession of this diaphragm is perhaps associated with the 

 habit of renewing the hydranth. The wound resulting from 

 the casting off of the old hydranth Avould be reduced to a 

 minimum, and healing could very rapidly be effected. 



Immediately below the hydranth can l)e seen the perisarc 

 groove (fig. 5, p.g-). An enlarged view is shown in fig. 6. 

 The secretory cells [p. s. c.) are richly granular, and towards 

 the free surface the perisarc substance can be observed to 

 run in for some little distance between them. The perisarc 

 groove is shown in horizontal section in fig. 13. 



The modification of the endoderm in the hydrocaulus is 

 considerable. In the middle region a longitudinal banding, 

 due to the presence of endodermal canals, can readily be seen 

 in the preserved specimen (fig. 4). This banding is more 

 conspicuous in the living animal, and can be traced up to the 

 hydranth, in the immediate neighbourhood of which they 

 , become closely applied to each other. 



Fig. 10 is a transverse section of the middle region, showing 

 in this case sixteen endodermal canals. A reticulum of fine 

 branching endoderm cells with large nematocysts occupies 

 the greater part of the coelenteron. An enlarged view of a 

 couple of canals is seen in fig. 15. Each canal consists of an 

 outer wall of regular columnar epithelium on which no cilia 

 could be detected with certainty. The inuer surface of this 

 epithelium is concave, and the canal is completed by branch- 

 ing cells (c. c.) forming a thin roof. 



These canals anastomose to some extent, and at the upper 

 end of the hydrocaulus they fuse into some nine to twelve 

 canals. At some little distance below the diaphragm the 



