70 HYDROZOA. 



branch is complex, consisting of two or three 

 distinct portions, these are gradually produced 

 as each of the bud-like processes elongates. "The 

 involucrum is formed as a process of the ectoderm 

 of the distal end of the peduncle. In Physophora, 

 the distal end of the peduncle itself undergoes a 

 singular dilatation, and helps to form the envelope 

 for the sacculus." 



The development of the gonophores has, in the 

 account given of their structure, been sufficiently 

 described. That of the nectocalyces is, at first, 

 precisely similar, but the central mass does not, in 

 these, give rise to a manubrium, while the cen- 

 tral cavity, into which the longitudinal canals open, 

 remains very much larger. 



The hydrothecae of the SertularidcG are formed 

 by the gradual separation from the body of each 

 pol}T3ite of the outer layer, or pol3rpary, excreted 

 b}^ its ectoderm, which, opening distally, displays 

 the cup-shaped cavity, characteristic of different 

 species. 



The relative succession of the appendages also 

 demands attention. In the fixed Hydrozoa the 

 distal polypites are first developed, whereas the 

 proximal appendages are the youngest in the 

 PhysophoridcB and Calycophoridce. But this rule 

 does not appear to govern the nectocalyces in the 

 last-mentioned group, their precise order of deve- 

 lopment still remaining involved in some com- 

 plexity. 



The phenomena indicated in the preceding 

 brief detail of the life-history of the Hydrozoa 

 have, in addition to their special value, a wider 

 interest for the philosophic student of zoology. 



