METAZOA COELENTERA. 115 



AuJopora serpens Goldf. (No. 157) begins as a single form, 

 then by budding, a creeping, irregular colony is produced. 

 The origin and structure of a compound coral is illustrated 

 by Pleurodictyum lenticulare Hall (PL 158, figs. 1-9). 

 The first coral animal that started the colony was single. 

 Its skeleton was shaped like an inverted cone smooth in 

 the earliest and ribbed in the later stage (figs. 1,2, 3). 

 At first the interior is simply granulose. but above the 

 middle portion the granules are arranged in rows which 

 are probably the beginnings of walls. When limy walls 

 exist in the Alcyonaria, they riiay be called pseudosepta, 

 since the true septa of the Hexactiniae correspond in 

 disposition and number to the fleshy mesenteries, which 

 cannot be said of the walls of the Alcyonaria in general. 

 These rudimentary walls are without pores and the whole 

 skeleton is covered by an external limy layer, the epitheca. 



Thus it is seen that the primitive ancestral form of corals, 

 as proved by this first stage of the compound coral, is 

 extremely simple, imperforate, and without tabulae or 

 well developed walls. In the next stage a bud appears 

 which is in direct communication with the parent form, 

 giving rise to an opening or pore in the wall of the latter 

 (fig. 4). This is the Aulopora stage which is seen more 

 clearly in fig. 5. Some doubt exists in regard to this 

 figure, but in all respects excepting the position and direc- 

 tion of the bud, this form agrees with Pleurodictyum lenti- 

 culare and may therefore be regarded as one of its early 

 stages. The buds appear alternately (figs. 6, 7, 8) until 

 a circle surrounds the original form (fig. 9). The second 

 stage is now completed. The colony increases twice in 

 diameter until the mature condition is reached. 



The origin and growth of the colony of Michelinia con- 

 vexa d'Orb., are shown diagrammatically in PL 159, figs. 

 1-7. In fig. i the first corallite is represented in the cen- 

 ter with its circle of corallites which have arisen alter- 

 nately, as in Pleurodictyum lenticulare. Michelinia, how- 

 ever, advances farther than the last named coral and 



