HYDROZOA. 



233 



Usually the colony is strengthened and protected by the perisarc, 

 a cuticular tubular secretion of the ectoderm. In some (fig. 175) 

 the perisarc stops at the base of the hydranth; in others (fig. 176) 

 it expands distally into a wide-mouthed bell, the hydrotheca, into 

 which the hydranth may retract at times of danger. In rare cases 



FIG. 176. Campanularia geniculata. eft, ectoderm; en, entoderm; p, perisarc, ex- 

 panded around hydranth to a hydrotheca; s, supporting layer. 



this perisarc may be greatly increased and calcified, forming large 

 coral-like masses with openings from which the hydranths may 

 protrude (fig. 177). 



FIG. 177. A bit of Millepora alcicornis, enlarged. (After Agassiz.) 



The lack of sexual organs, which distinguishes the marine 

 species from the fresh-water Hydra, is explained by the fact that 

 sexual individuals of special form are produced from the colony 



