730 THE ANIMAL KINGDOM. 



the mouth, have them on the surfaces turned to its cavity. The first is 

 termed the dorsal chamber, the second the ventral. The three lateral 

 chambers on each side have consequently each a single muscle projecting 

 into their cavities from the surfaces of the mesenteries turned towards the 

 ventral aspect of the zooid. The two mesenteries inclosing the dorsal 

 chamber are frequently of great length, and in the bud they usually 

 develope more rapidly than the remaining mesenteries. The two mesen- 

 terial filaments of the dorsal mesenteries are composed of a border or 

 ridge of high columnar cells, each with a single powerful cilium backed by 

 flattened endodermic cells such as cover the surfaces of the mesenteries. 

 The columnar cells are derived from a down-growth of the lower edge of 

 the oesophagus ; they are therefore ectodermic. Their cilia always pro- 

 duce an ascending current l . The mesenterial filaments of the remaining 

 mesenteries are cpmposed entirely of endoderm cells which become 

 columnar and for the most part glandular, but possess a single cilium. 

 Thread cells of minute size occur among them. They develope before the 

 dorsal filaments in the zooid originating from the egg. The sexes appear 

 to be separate as a rule, and the colonies are even of one sex. In Coral- 

 Hum, however, hermaphrodite individuals may occur, and the zooids of a 

 single branch or of neighbouring branches in a colony may be of different 

 sexes. The genital products are attached to the faces of a mesentery and 

 possibly of particular mesenteries, in pedunculate capsules, always few in 

 number. They are derived from the endoderm. 



The autozooid always has the typical structure above given. The si- 

 phonozooid differs from it in the absence of tentacles and retractor muscles ; 

 in the great development of the siphonoglyphe ; in having the two dorsal, 

 and sometimes the two ventral mesenteries, longer than the rest, the first 

 named only having mesenterial filaments 2 ; and in being sexless. Such 

 zooids occur in Sarcophyton and Heteroxenia among Alcyonidae, and in 

 Pennatulidae ; among Pseudaxonia in Paragorgia and Siphonogorgia where 

 they bear ova, and also in Coral Hum where it is stated that they develope 

 into autozooids. Similarly certain zooids found in the Pennatulids 

 Halisceptrum and Virgularia, which differ however from siphonozooids in 

 having genital organs and traces only of the dorsal mesenterial filaments, 

 develope into autozooids when they have discharged their sexual function. 



1 The only filaments present in Xenia and Sympodium are the two dorsal. Haacke, Z. A. vii. 

 1884. 



2 Wilson says (op. cit. p. 725) that the siphonozooids of Renilla have no mesenterial filaments. 

 According to Professor M. Marshall (Trans. Roy. Soc. Edinburgh, xxxii. p. 145) the siphonozooids of 

 Umbellula gracilis possess a single tentacle ; and the larger siphonozooids of Pennat^da phosphorea 

 var. acukata have a long abaxial process formed probably by elongated calyx teeth (op. cit. p. 126). 

 A single large siphonozooid terminates the axis of the colony in Renilla. Water is constantly dis- 

 charged by it ; hence ' exhalent zooid ' = Haupt- zooid of Kolliker. The other siphonozooids are 

 small, clustered and inhalent. 



