PHYSIOLOGY OF THE INVERTERRATA. 413 



covered by a delicate chorion, and contain a variously coloured 

 vitellus with germinal vesicle and spot ; the ova also contain 

 a little albumin. The spermatozoa are nearly always com- 

 posed of a round or oval body and a delicate hair-like filament. 

 " With a few exceptions, the embryo leaves the egg as a bi- 

 laterally symmetrical larva, provided with ciliated bands, and 

 otherwise similar to a worm-larva, which may be termed an 

 Echiiwpmdiwm. The conversion of the Echinopcedium into an 

 Echinoderm is effected by the development of an enterocoele, 

 and its conversion into the peritoneal cavity and the ambu- 

 lacral system of vessels and nerves ; and by the metamorphosis 

 of the mesoderm into radially disposed antimeres, the result 

 of which is the more or less complete obliteration of the 

 primitive bilateral symmetry of the animal." (Huxley.) 



The external appearances of the sexual organs in the 

 EcMnodermata are somewhat similar, but at the period of pro- 

 creation they frequently differ in colour. They are composed 

 of either simple or branched tubules with or without excretory 

 ducts. In the latter case, the contents of the organ, male or 

 female as the case may be, are discharged by rupture into 

 the body cavity, from whence they pass out through the 

 respiratory openings. 



The EcMnodermata are devoid of copulatory organs; the 

 ova being fecundated by the spermatozoa in the water in 

 which these animals live. 



In the Holothuridea, there is only one testis and ovary in 

 male and female respectively.. Both are composed of a tuft 

 of highly-branched tubules, which unite to form a common 

 duct, which opens externally near the mouth. The early 

 stages in the development of Holothuria are like those of the 

 Asteridea, which will be described later in this chapter.. The 

 free-swimming larva is called an Aiiricvlaria. The larva is 

 transparent, vermiform, and has four or five bands of cilia ; 

 and while still growing, the young Holothurian begins to 

 .bud out by the side of the larval stomach. The larva 

 or Auricularia is gradually absorbed by the developing 



