BRITISH ZOOPHYTES. 31 



zooid enters upon a state of quiescence, and dissolves 

 away in the process of fulfilling its mission. 



The medusiform zooids were for a long time con- 

 sidered as distinct animals, but the life-history of a 

 number of species has been worked out by many 

 careful observers. 



After the liberation of the ova and its fertilization, 

 whether the liberation has been from a fixed or a 

 free gonozooid, the usual process of segmentation 

 commences which results in the development of a 

 free ciliated larva, called by Dalyell a jplanula. This 

 planula (Plate I. fig. 7) may be defined as a closed 

 ciliated hollow sac with double walls. After several 

 days, during which it swims about freely^ it drops its 

 cilia and becomes extended at one end into a kind of 

 base or disc, by which it attaches itself to some object. 

 It now becomes covered with a chitinous layer within 

 which, at the upper extremity, the tentacles are deve- 

 loped. After a time this layer is ruptured and the 

 tentacles liberated, a mouth also being now formed^ 

 and the typical form of the species is ultimately 

 assumed. As before stated, in Hydra the planula stage 

 is suppressed. This was also thought to be the case in 

 Tubularia, but recent researches seem to show that 

 this genus presents no diversity from the regular 

 mode of development. 



The presence of a nervous system or sense organs 

 in the Hydroidahas been often sought for, and recently 

 not without success. Prof. Allman, in the Annals and 

 Mag. of Nat. Hist, for November, 1874, and afterwards 

 in the Proc. of the Royal Society for February 11, 1875, 

 describes the structure of M. Phrygia (If. Gocksii), 

 and states that between the ectoderm cells proper and 



