ANATOMY — CLASSIFICATION 



567 



The so-called blood system is in its main features similar to 

 that of Echinoidea. It consists of a blood-ring surrounding the 

 oesophagus inside the water-vascular riiig, and sending branches 

 along the stone -canal, and of dorsal and ventral strands accom- 

 panying the gut in its course. These are best marked in the 

 region of the intestine, where absorp- 

 tion principally takes place ; in the 

 wall of the stomach they are repre- 

 sented by a delicate plexus which 

 can hardly be traced into connexion 

 with the blood -ring. The dorsal 

 "vessel" is situated in a fold of 

 peritoneum projecting from the intes- 

 tinal wall ; it gives off branches to 

 the intestine, which unite on its 

 surface to form a plexus. In the 

 middle limb of the intestine these 

 branches are grouped into tufts, and 

 the fold of peritoneum between 

 successive tufts becomes absorbed ; 

 through the holes so formed branches 

 of the respiratory tree penetrate, so 

 that the trees cannot be separated 

 from the intestine without tearing 

 the dorsal vessel (Fig. 256, is). 



The genital organs consist of a single group of branched 

 tubes situated on the left side of the dorsal mesentery, which 

 converge to open into a short genital duct, which leads to a pore 

 situated in the mid-dorsal line, a short distance behind the 

 feelers. From the common point of origin of the tubes, the 

 "genital base," as it is called, a worm-shaped genital stolon^ 

 extends back along the genital duct towards the body-wall. 

 There is no genital rachis. 



Fig. 257. — Types of calcareous rings 

 and of ossicles. A, calcareous 

 ring of Phyllophorus rugosus, 

 X 2 ; B, calcareous ring of Holo- 

 thuria cinerascens ; C, ossicle of 

 Holothuria atra ; D, ossicle of 

 Jlolothuria/nsco-ntbra. r, Eadial 

 piece. (After Ludwig.) 



Classification of Holothuroidea. 



The class is in many points of structure exceedingly variable, 

 but many striking variations in important organs occur in allied 



1 Hjalmar Thdel, "On a singular Case of Hermaphroditism in Holothurids," 

 Bih. Svenska Vet. Akad. Hand, xxvii. Af. 4, No. 6; 1901. 



