100 TWENTY-FIRST REPORT. 



the upper ectoclenn of the coenosarc, and the endoderm of the tubes. In this 

 way the gastral cavities of all members of the colony are placed in direct 

 communication with each other. 



At intervals the skeleton projects above the coenosarc, forming conical, 

 smooth spines and spinules. These sometimes form bridges of chitin over an 

 intercommunicating tube, which led Mr. Hincks to conclude that the chitinous 

 covering existed above, as well as below the coenosarc (PI. 2, Fig. 4). 



Carter (1873) tells of a specimen in the British Museum in which the 

 whole of the shell has become transformed into the horn-like skeleton of 

 Hydractinia. From the smooth internal appearance, he infers that the shell 

 had been tenanted by an Eupagurus, which left after the entire shell had been 

 transformed. 



HISTOLOGY. 



The lower ectoderm of the coenosarc is composed of long, slender cells, 

 quite irregular in shape. They are more or less vacuolated, and contain a 

 single nucleus situated near the center of the cell. The nucleus is oval in 

 shape, and contains several nucleoli. It is the function of this layer to secret, 

 extend and renew the chitinous skeleton. 



The upper ectoderm of the coenosarc is formed of a single layer of cells 

 more regular in size, and more cubical in shape. Nematocysts are occasionally 

 present in this layer (PI. 2, Fig. 4). 



This layer is continuous with that of the polyps, the main difference in 

 the latter being a greater variation in shape of their cells. 



The endoderm of the coenosarc is made up of a single layer of cubical cells 

 containing a single, oval niicleus in their center. 



In the gasterozooids the endoderm contains long, narrow cells, which vary 

 in length so that the free ends are not at the same level. In this way longi- 

 tudinal ridges are formed in the lumen, which in cross section present a very 

 irregular appearance (PI. 2, Fig. 3). As this is especially characteristic of 

 the nutritive polyps, it is evident that even in this low form of life the rudi- 

 mentary alimentary canal is thrown into elevations for the increase of surface. 



These cells are wider at their free ends, and are vacuolated. Their nuclei 

 are oval, are situated near the middle region of the cell, and possess one or 

 more nucleoli. 



The endoderm of the blastostyles is composed of long, narrow ciliated 

 cells. In the head region these often contain several nuclei, but in the body 

 they possess a single, large nucleus. 



The endodermal cells of the dactylozooids are approximately equal in size. 

 They usually contain many vacuoles, and a single nucleus, which is situated 

 in the middle of the cell. 



The tentaculozooids possess long, narrow cells, which are more regular in 

 size and shape than those of the other polyps. 



