THE ANTHOZOA 13 



usually consists of spicules, which may be fusiform, club-shaped, 

 cross-shaped, or discoid; they are seldom smooth, but generally 

 covered with spines or warty projections. They are developed 

 within ectodermal cells, and are therefore entoplastic products. 

 Most commonly the spicule-forming cells pass out of the ectoderm 

 and are imbedded in the mesogloea, but Bourne (9) has shown 

 that in the genus Xenia the spicule-forming cells remain in the 

 ectoderm ; this is also the case in some members of the genus 

 Clavularia. In one Alcyonarian (Heliopora coerulea) the calcareous 

 skeleton is not spicular but lamellar, like that of Madreporarian 

 corals ; it is formed by a special layer of cells called calicoblasts, 

 derived from the ectoderm. 



An organic horny skeleton is frequently present, either in the 

 form of an external horny investment (Cornularia), or of an in- 

 ternal axis, as in Pennatula, Gorgonia, and others ; or there may 

 be a half horny half calcareous axis, as in Isis ; or there may 

 be an axis formed of calcareous spicules imbedded in horny sub- 

 stance, as in many Pseudaxonia. 



The development of the Alcyonaria has been studied by Kowa- 

 levsky and Marion (69), E. B. Wilson (96), and von Koch (61). 

 The segmentation of the ovum is complete, and results in the 

 formation of a solid morula. Wilson has shown that in Renilla 

 the ovum divides at once into many, usually sixteen, blastomeres. 

 As neither von Koch nor Kowalevsky and Marion found earlier 

 stages of segmentation, this exceptional mode of division may 

 possibly be the rule amongst the Alcyonaria. After repeated sub- 

 division of the blastomeres of the sixteen cell stage, the solid 

 mass of cells is divided into two layers an external ectoderm and 

 a central mass, the primitive endoderm. The coelenteron is formed 

 by the dissolution and absorption of the central cells of the endo- 

 dermic mass, the disintegrated cells being engulfed by and serving 

 as nourishment for the more peripheral cells which become the 

 definitive endoderm. There is no gastrula stage in Clavularia, 

 Gorgonia, or Kenilla, though Haeckel has described a gastrula in 

 the case of Monoxenia. The embryo, at the time of the forma- 

 tion of the coelenteron, becomes pear-shaped, the ectoderm cells 

 become columnar and acquire cilia, and the larval stage known 

 as a planula is reached. The planula escapes from the cavity of 

 the parent zooid, in which the earlier stages of development have 

 proceeded, and swims freely in the water by means of its cilia. 

 There is, as yet, no communication between the coelenteron and 

 the exterior. After a free existence of shorter or longer dura- 

 tion, the embryo fixes itself by one end of its elongate body, and 

 a stomodaeum is formed at its opposite extremity by invagination 

 of the ectoderm. At the bottom of the invagination a perforation 

 places the coelenteron in communication with the exterior. The 



