480 THE ANATOMY OF INVERTEBRATED ANIMALS. 



ral face of the body, around the anus, giving off various 

 branches to the viscera, and communicating with the so-called 

 heart, are mere subdivisions of the interval between the 

 parietes of the body and those of the alimentary canal, aris- 

 ing from the disjDosition of the ambulacral vessels and that of 

 the walls of the peritoneal cavity ; both of which, as their 

 development shows, are the result of the metamorphosis of 

 saccular diverticula of the alimentary canal, which have en- 

 croached upon, and largely diminished, the primitive perivis- 

 ceral cavity which exists in the embryo. 



The peritoneal cavity of the body and rays is filled with 

 a watery corpusculated fluid ; a similar fluid is found in the 

 ambulacral vessels, and probably fills all the canals which 

 have been described. The corpuscles are nucleated cells, 

 which exhibit amoeboid movements ; and the fluid so obvious- 

 ly represents the blood of the higher animals, that I know not 

 why the preposterous name of " chylaqueous fluid " should 

 have been invented for that which is in no sense " chyle," 

 though, like other fluids of the living body, it contains a good 

 deal of water. As the cavities- of the tubular caeca of the 

 perisoma communicate freely with the general cavity, and 

 their walls share in the general ciliation of the lining of the 

 cavity, it is very probable that they may subserve the func- 

 tion of respiration. 



The genital glands are situated in pairs, interradially, at 

 the junction of the body with the rays. Each gland is di- 

 vided into a number of elongated processes, the common base 

 of which is attached to the face of one of the interradial septa, 

 while the processes project freely into the cavities of the arms. 

 According to Hoffmann and Greef, the inner cavities of the 

 genital processes are filled when the vascular system is inject- 

 ed. It is possible, therefore, that the genital glands are mere- 

 ly processes of the mesodermal layer, in the walls of which 

 the genital products are developed ; in which case there would 

 be a close approximation between the genital glands of the 

 Star-fishes and those of the Crinoids. According to Greef, 

 the external openings of the genital glands are visible in 

 Uraster, in the breeding-season ; in other Star-fishes, they are 

 conspicuous in the interradii of the aboral face of the body. 

 In Z/uidea, Ophidiaster, and some other genera, the glands 

 extend far into the interior of the arms ; and Prof. G. O. Sars ' 



1 'j Researches on the Structure and Affinity of the^ Genus Brisinga," 1870. 

 In this important memoir the author proves that Brisinga is a true Asterid, 

 and not, as has heen supposed, a transitional form between the Asteridea 

 and the Ophiuridea. 



