I 



CNIDARIA 87 



these at the periphery, it follows that the oldest individuals 

 are more and more crowded toward the dorsal side (Fig. 38). 

 The terminal polyp also shares this fate. In this way is 

 developed a discoid colony, the marginal individuals of which 

 are the youngest. 



The zooids are formed at the same time as the sexual 

 polyps. Even immediately after the appearance of the first 

 pair of polyp buds, a large termi7ml zooid (Fig. 37 z) can be 

 recognized ; this functions as an excurrent opening, and is 

 soon followed by the so-called marginal zooids (mz), arranged 

 in two lateral dorsal rows, while dorsal zooids {dz) make their 

 appearance on the dorsal side of each of the individual 

 polyps. 



As far as the development of Pennatula is at present 

 known, it is strikingly similar to that of Renilla. Lacaze- 

 DuTHiERS (No. 90) has made some statements on the earliest 

 stages of Pteroides (Pennatula) griseum; the later stages, 

 relating to budding, have been described by Jungersen (No. 

 81). Here also we find lying at the foundation of the colony 

 an axial individual which is retained for a considerable time 

 as the terminal polyp, and on the sides of which bud forth 

 the daughter individuals, which appear in pairs, but alter- 

 nating with one another. At the bases of these lateral 

 polyps, that are the first to appear, and in positions corre- 

 sponding to the ventral side of the axial individual, new buds 

 continue to arise, thereby introducing the development of 

 the pinnate leaflets, of which accordingly the dorsal indi- 

 vidual exhibiting the greatest length is the oldest. On the 

 dorsal side of the axis we find an unpaired terminal zooid 

 and other zooids which are arranged in two rows. The 

 lateral zooids, which belong to the ventral surface, are not 

 developed until later. In the young stages the terminal 

 zooid probably functions as the only excurrent opening. In 

 the older stages, on the other hand, there is found at the 

 upper end of the axis a group of apical zooids, among which 

 are probably to be found the terminal zooid and the degene- 

 rated terminal polyp, as well as the adjoining polyps, these 

 having assumed the function of the terminal zooids. 



In the peduncular septum^ which here also divides the gas- 



