ACTINOZOA. 91 



CLASS II. THE ACTINOZOA, 



Codenterates icith a digestive sac partially free from the body-cavity open' 

 ing into it beloic and field in place by six or eight mesenteries radiating from 

 the digestive cavity and dividing the pericisceral space into chambers. Mouth 

 surrounded icith a circle of tentacles, which are hollow, communicating di- 

 rectly with the perivisceral chambers. A slightly marked bilateral symmetry. 

 To the edges of the mesenteries (usually the free ones) are attached the repro- 

 ductive glands, both male and female, or of one sex alone ; also the craspeda, 

 or mesente rial filaments, ichich contain a large number of lasso-cells. Body 

 either entirely fleshy, or secreting a calcareous or horny coral-stock, and 

 when the species is social connected by a ccenenchyme. In some forms (sea- 

 pens) the entire colony capable of limited locomotion. No well-marked 

 nervous system, but a plexus of fusiform ganglionic cells connected by nerve- 

 fibres in the base of Actinians. Reproduction by self division, gemmation, 

 or by ova, the sexes being separate or united in the same individual; the 

 young undergoing a morula and gastrula condition, and then becoming 

 fixed. 



Order 1. Zoantharia. Mesenteries and tentacles usually six or in mul- 

 tiples of six, corallurn with calcareous septa. Meseuterial fila- 

 ments abundantly developed (Astrsea, Madrepora, Actinia). 



Order 2. Alcyonaria. Mesenteries and tentacles always eight in num- 

 ber. Coral-stock without true septa. Mesenterial fila- 

 ments not usually numerous. Corallum usually horny, and 

 the whole colony in the Pennatulacea capable of locomo- 

 tion (Alcyonium, Gorgonia, Pennatula, Renilla). 







VIEW OF THE CLASSIFICATION OF THE ACTINOZOA. 



Alcyonaria. 

 (Alcyonium.) 



Zoantharia. 

 (Actinia.) 



ACTINOZOA. 



Laboratory Work. Verrill has preserved Actinia? completely ex- 

 panded by slowly adding a saturated solution of picric acid to a small 

 quantity of sea-water in which they had expanded. AVhen dead they 

 should be transferred to a pure saturated solution of the acid, and 

 allowed to remain for from one to three hours, according to size, etc. 

 They should then be placed in alcohol, which should after a day or two 

 be renewed. Thus hardened they can be cut into sections. Corals 

 can be studied by grinding or sawing sections, and, if desirable, treated 

 as in the case of the corallum of the Millepores. 



