70 



HYDEOZOA. 



branch is complex, consisting of two or thre 

 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 Physophm 

 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 hydrothecse of the Sertularidce are formed 



by the gradual separation from the body of each 



polypite of the outer layer, or polypary, excreted 



by its ectoderm, which, opening distally, displays 



the cup-shaped cavity, characteristic of different 

 species. 



The relative succession of the appendages also 



demands attention. 



Hyd 



distal polypites are first developed, whereas the 

 proximal appendages are the youngest in the 

 Physophoridce 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 



zoa 



ider 



Hydr 



have, in addition to their special value, a w»~ 

 interest for the philosophic student of zoology: 









