EXPI^ANATION OF PLATE XVII. 



Fig. I. — Transverse section of the lower part of a hydranth. 

 CC ^= Central cavity. M = Muscle strands. 

 The '' mesenteries " which unite the lining of the 

 central cavity to the ectoderm lie in a shrinkage space. 



Fig. 2. — A somewhat oblique section at a higher level through 

 another hydranth. Four '' mesenteries " are seen ; 

 those on the left are cut obliquely. 



Fig. 3. — A longitudinal section of a hydranth through the oral 

 aperture. 



The outlines for figs, i, 2 and 3 were drawn under 

 ^-inch objective with the camera lucida. The muscle 

 fibres are represented conventionally by dots and black 

 lines. The real appearance is of a delicate fibrillation 

 more accurately portrayed in fig. 4. 



Fig. 4. — A small portion of a section from the same series as 

 the one shown in fig. 2 under y'^-inch objective, 

 Ec = Ectoderm. CC = Central cavity. M = Muscle 

 fibrils. 



This figure shows that each " mesentery " is composed 

 of two layers of delicate, nucleated cells which sepa- 

 rate to form the central cavity. These features are 

 best seen in the mesentery on the right ; the one on 

 the left being cut somewhat obliquely. The charac- 

 ters of the "^ specialized endoderm cells " are shown ; 

 the well-marked outlines, the peripheral granules, and 

 the absence of nuclei. 



Fig. 5. — A small portion of the disc in section under yV-inch 

 objective. 



On the right a " straight tube " is seen opening into 

 the common spaces of the disc. 



The ectoderm is seen dipping down among the other 

 layers : this is not usual. 



