134 ECHINODERMATA 



canal of the water vascular system produce ameboid lymphocytes 

 that may be quite important. 



The perihaemal system, compared by some to a true blood vas- 

 cular system, consists of the axial organ (genital stolon), adjoining 

 the stone canal; the oral ring vessel, surrounding the mouth and 

 divided by a septum; the perihaemal vessel, divided by a septum, 

 and the five radial blood vessels that are found in the rays. The 

 perihaemal septum is found to contain gelatinous connective tissue 

 and many white blood corpuscles (leucocytes), and is perforated by 

 many irregular channels. 



Excretion. — Besides osmotic excretion, the starfish has the ability 

 to excrete shells and other wastes from the mouth. Indigestible 

 foods also pass through the intestine and out at the anal aperture. 

 The ameboid corpuscles of the coelomic fluid aid in excretion. 



Reproduction. — Starfishes are not hermaphroditic, although some 

 other Echinoderms are. Each animal produces from paired gonads 

 either eggs or sperms. The gonads are situated in the rays with 

 their ducts opening on the aboral surface through minute pores on a 

 pair of sieve-like plates situated close to the bases of the arms, 

 between the rays. During the spawning season the gonads may 

 have so many eggs that the starfish will have enormously distended 

 rays, and the hepatic ceca may be crowded until they are much 

 reduced in size. The eggs are fertilized outside the body, although 

 many perish without fertilization having been effected. In one 

 year a starfish may have arms two and one-half inches long and be 

 ready to spawn. 



Artificial Parthenogenesis. — Norman, Greene, Matthews, Mor- 

 gan and Loeb, developed a method of inducing the development of 

 unfertilized eggs of echinoderms. Loeb continued the work for 

 many years and proved that in the absence of sperms, variation in 

 the temperature, the addition of sodium chloride, potassium 

 bromide and cane sugar solutions would cause normal larvae to 

 develop. For years students in the embryology course at the 

 Marine Biological Laboratory at Woods Hole, Massachusetts, have 

 repeated the experiments with complete success. Subsequent to 

 his echinoderm work, Loeb succeeded in producing fatherless frogs. 

 (See page 299.) In nature, echinoderms and some annelids as well 

 as plant lice and rotifers normally develop parthenogenetically, that 

 is, without the stimulus of sperm. 



Nervous System. — The nervous system consists of the nerve ring. 



