THE ANTHOZOA 



33 



enormously larger than the distal portion, and forms the peduncle and 

 rachis of the colony, its cavity being divided by the septum into a 

 prorachidial (asulcar or ventral of Kolliker) and a metarachidial (sulcar or 

 dorsal of Kolliker) chamber. The distal portion of the mother zooid 

 becomes at an early period nothing more than a relatively minute 

 appendage upon the upper part of the stem which has been developed 

 from it. At the base of the distal or calycine portion of the mother 

 zooid, a bud, formed on the asulcar side, forms the first or terminal 

 siphonozooid. The lateral zooids are formed as buds on either side of 

 the terminal zooid, and as each is developed a siphonozooid is formed at 

 its base. The pinnae are formed by the development of secondary buds 

 at the bases of the primary pararachidial autozooids. In the course of 

 growth the proximal portions of the rows of autozooids so formed become 

 fused together, the distal ends remaining free and forming small calices, 

 strengthened by a crown of eight points formed by spicules, and the 

 tentacular portions of the zooids are retractile within the calices. 



The development of the Pennatulid colony and the formation of the 

 peduncular septum will best be understood by a study of Fig. XVII. 

 The existing families of the Pennatulaceae appear to have diverged from 

 an ancestral form resembling Protocaulon molle. The lines of divergence 

 may be briefly indicated as follows : From an original form in which 

 simple sessile autozooids, each with a siphonozooid at its base, were 

 borne on either side of an axial zooid, differentiated into peduncle and 

 rachis. (1) The autozooids have become more numerous, have encroached 

 on the whole surface of the rachis, and the siphonozooids, multiplying in 

 number, have filled up the spaces between the autozooids. Such a 

 condition is found in the Veretillidae, in which a bilateral symmetry is 

 replaced by a radial symmetry. (2) The autozooids, whilst increasing in 

 number, are confined to two opposite aspects of the rachis, and there 

 form, at first indistinct, afterwards distinct rows. The siphonozooids 

 also increase in number, and lying between the bases of the autozooids, 

 occupy the remainder of the pararachidial surfaces. From this condition, 

 realised in the Funiculinidae, differentiation proceeds in two directions, 

 (a) The autozooids are confined to the upper part of the rachis, and 

 are finally grouped in an umbel at its summit, the remainder of the 

 rachis bearing siphonozooids only on the pararachides, e.g. the Umbell- 

 ulidae. (J3) The autozooids are disposed in oblique rows on the 

 pararachides, and their proximal portions are fused so as to form leaf-like 

 appendages of the rachis or pinnules. In the family Virgularidae the 

 autozooids are short and the pinnules are small and inconspicuous, in the 

 Pennatulidae the autozooids are much elongated and form conspicuous 

 pinnules. The family Gondulidae is derived from the Pennatulidae by 

 suppression of the peduncle, the colony, consisting of rachis and pinnules, 

 being fixed by the proximal end of the rachis. The family Renillidae 

 appears to have branched off from the Umbellulid stem ; the peduncle is 

 short, the rachis is much expanded and forms a kidney-shaped expansion, 

 bearing on its upper surface numerous irregularly distributed autozooids, 

 amongst which are situated groups of siphonozooids. The following 

 classification of the Pennatulacea is founded on Kolliker's work, but is 



