414 PHYSIOLOGY OF THE INVERTEBRATA. 



Holotliurian ; and the adult form of the animal is attained 

 without any further changes. 



Although the sexes are distinct, there is one exception 

 among the Holotlmridca^ and that is the genus Symqita. 

 These animals are hermaphrodites. According to De Quatre- 

 fages,* the testes and ovaries are united so as to form one 

 organ. This organ consists of branched tubules and secretes 

 both spermatozoa and ova ; its excretory duct opens near the 

 oral end of the body. 



In the Astcridea, the sexes are separate. The genital 

 organs are very similar in appearance, but the colour of the 

 ova is either yellow or red and the seminal fluid is white. 

 The ova are fecundated in the water. There are five pairs of 

 genital glands, one pair lying in each ray. They are saccu- 

 lated or racemose organs, whose ducts open externally by a 

 narrow orifice on the dorsal side of the body. Dr. G. 0. Sars 

 has shown that in Bruin(ja cndccacncmos the genital organs 

 consist of many distinct glands forming two well-marked 

 series, which are situated one on each side of the middle line 

 of the central half of each ray. 



Concerning the development of Asterias (a typical example 

 of the Asteridea), the following may be taken as an outline 

 of what occurs : — (a) The ovum, after fecundation, becomes 

 totally and equally segmented — thus forming the morula 

 stage. (&) The segmented ovum gives rise to a spheroidal 

 embryo consisting of an external ciliated cellular wall and an 

 internal gelatinous substance, (c) A depression of the ex- 

 ternal wall now makes its appearance, and gives rise to the 

 first rudiment of the alimentary canal. The opening of this 

 depression ultimately becomes the anus. This is the gastrula 

 stage. (<■/) The ciliated embryo lengthens and four surfaces 

 can now be distinguished. There is a continuous band of 

 cilia which has a locomotory function. The alimentary 

 canal, which in this stage acquires a mouth, becomes modified 

 into three portions — oesophagus, stomach, and intestine. 

 * Annales des Sciences Naturelles, tome 17, p. 66. 



