140 THE AX ATOMY OF INVERTEBRATED ANIMALS. 



Actinozoa develop skeletons, the corallites may be distinct, 

 and connected only by a substance formed by the calcifica- 

 tion of the ccenosarc, which is termed ccenenchyma ; or the 

 thecae may be imperfectly developed, and the septa of adja- 

 cent corallites run into one another. There are cases, again, 

 in which the calcareous deposit in the several polyps of a 

 compound Actinozoon, and in the superficial parts of the cce- 

 nenchyma, remains loose and spicular, while the axial por- 

 tion of the ccenosarc is converted into a dense chitinous or cal- 

 cified mass — the so-called sclerobase. 



The mesoderm contains abundantly developed muscular 

 fibres. The question whether the Coralligena possess a ner- 

 vous system and organs of sense, hardly admits of a definite 

 answer at present. It is only in the Actinidw that the ex- 

 istence of such organs has been asserted ; and the nervous 

 circlet of Actinia, described by Spix, has been seen by no 

 later investigator, and may be safely assumed to be non-exist- 

 ent. Prof. P. M. Duncan, F. R. S., 1 however, has recently 

 described a nervous apparatus, consisting of fusiform gan- 

 glionic cells, united by nerve-fibres, which resemble the sym- 

 pathetic nerve-fibrils of the Vertebrata, and form a plexus, 

 which appears to extend throughout the pedal disk, and 

 very probably into other parts of the body. In some of the 

 Actinidce (e. g., Actinia mesembryanthemum), brightly-col- 

 ored bead-like bodies are situated in the oral disk outside 

 the tentacles. The structure of these "chromatcphores," or 

 ''bourses calicinales," has been carefully investigated by 

 Schneider and Rotteken, and by Prof. Duncan. They are 

 diverticula of the body wall, the surface of which is com- 

 posed of close-set " bacilli," beneath which lies a layer of 

 strongly-refracting spherules, followed by another layer of 

 no less strongly-refracting cones. Subjacent to these, Prof. 

 Duncan finds ganglion cells and nervous plexuses. It would 

 seem, therefore, that these bodies are rudimentary eyes. 



The sexes are united or distinct, and the ovum is ordina- 

 rily, if not always, provided with a vitelline membrane. The 

 impregnated ovum gives rise to a ciliated morula, which may 

 either be discharged or undergo further development within 

 the somatic cavity of the parent. The morula becomes a gas- 

 trula, but whether by true invagination or by delamination, 

 as in most of the Hydrozoa, is not quite clear. The gastrula 

 usually fixes itself by its closed end, while tentacles are de- 



1 " On the Nervous System of Actinia." (" Proceedings of the Royal Socie- 

 ty," October 9, 1873.) 



