ALCYONARIA DEVELOPMENT CLASSIFICATION 



341 



-Ec 



It is not known for certain when the fertilisation of the ova 

 is effected, but in Alcyonium digitatmn, and in the majority of 

 the Alcyonarians, it probably takes place after the discharge of 

 the ova from the zooids. A few forms are, however, certainly 

 viviparous, the larvae of Gorgonia capensis being retained within 

 the coelenteric cavity of the parent zooid until they have grown to 

 a considerable size. The other viviparous Alcyonarians are Coral- 

 Hum nohile (de Lacaze Duthiers), the " Clavulaires petricoles," 

 and Sympodium coralloides (Marion 

 and Kowalevsky), and three species 

 of Ne2^hthya found at depths of 

 269 to 761 fathoms (Koren and 

 Danielssen). The general features 

 of the development are very similar 

 in all Alcyonarians that have been 

 investigated. The egg contains a 

 considerable amount of yolk, and 

 undergoes a modified form of seg- 

 mentation. The free-swimming larva 

 is called a " sterrula." It consists 

 of an outer layer of clear ciliated 

 ectoderm cells, surrounding a solid fig. 

 endodermic plasmodium containing 

 the yolk. As the yolk is consumed 

 a cavity appears in the endoderm, and the larva is then called 

 a "planula" (Fig. 150). The mouth is subsequently formed 

 by an invagination of the ectoderm at the anterior pole. 

 The development of the mesenteries has not yet been fully 

 described. 



Classification. — The sub-class Alcyonaria may conveniently 

 be classified as follows : — 



150. — Ciliated "planula" larv; 

 of Alcyonium digitutum. Ec 

 Ectoderm ; End, endoderm. 



Grade A. Protalcyonacea. 

 Grade B. Synalcyonacea. 



Order 1. Stolonifera. 



Order 2. Coenothecalia. 



Order 3. Alcyonacea. 



Order 4. Gorgonacea. 



Order 5. Pennatulacea. 



