ECHINODERMA. 123 



the tubular feet ; wliieh are covered with so rich a network of vessels 

 that Valentin compares them with the lungs of tlie Salamander. 

 The chief office of these sacs, according to Tiedemann, is to pro- 

 trude, by contracting upon their fluid contents, the tubular feet, con- 

 tinued from them through the ambulacral pores ; but as the ter- 

 minal sucker of these feet is unquestionably perforated, Valentin * 

 rejects this explanation; he thinks the tubular feet imbibe the sea 

 Avater by their terminal pore, and convey it to tlie internal basal sac, 

 for the oxygenation of the blood, circulating over its parietes. 



The external branchiae are a more complicated form of respiratory 

 sac everted and extended ; they float in the external respiratory me- 

 dium, while the internal sacs receive it into their interior. 



The sea water can be admitted into the interior of the visceral 

 cavity through the interspaces of the teeth ; if it be actually intro- 

 duced by the tubular feet it must pass by exosmose through the 

 pores of the basal sacculi, which is contrary to analogy. 



Cuvier, Tiedemann, and Delia Chiaje have given more or less ac- 

 curate descriptions, but conflicting explanations, of the vascular system 

 of the Echinus. 



There is no doubt that the fusiform dilated contractile vesicle, 

 situated near the oesophagus, and surrounded by a double fold of 

 the mesentery, is the central organ or heart. Its cavity is subdivided 

 by muscular walls. From its oral end a trunk proceeds, which forms 

 a circle around the oesophagus at the base of the lantern, from which 

 the vessels of that part proceed. A second trunk is continued from 

 the opposite end of the heart, in the opposite direction, and forms 

 a corresponding circle around the anus. A vessel called the intes- 

 tinal artery runs along the concave margin of the intestine ; another 

 trunk called the intestinal vein accompanies the outer or convex 

 contour of the intestine, and receives many branches from the 

 membrane of the shell. The vascular circle round the anus (e), 

 receiving the veins of the ovaria, sends off five trunks which run 

 in the interspaces of the internal branchiae ; the capillaries of these 

 branchiae return into five other trunks, accompanying the preceding 

 five along the median interspace. One set must fulfil the office of 

 branchial arteries, the other that of branchial veins. The blood is of 

 a deep yellow colour; the blood-cells are granular and irregular, 

 but generally manifest a nucleus. 



Prof. Valentin, after a minute and searching scrutiny into the 

 anatomy of the vascular system of the Echinus, is unable to deduce 

 from that alone the course of the circulation. The ascertained facts 



* Valentin, p. 85. 



