206 LECTURE X. 



the salamander. The chief ofBce of these sacs, according to Tiede- 

 mann, is to proti'ude, by contracting upon their fluid contents, the 

 tube-feet, continued from them through the ambulacral pores ; but 

 as the terminal sucker of these feet is unquestionably perforated, 

 Valentin * rejects this explanation : he thinks the tube-feet imbibe 

 the sea-water by their terminal pore, and convey it to the internal 

 basal sac, for the oxygenation of the blood, circulating over its 

 parietes. The sea-vsrater can be admitted into the interior of the 

 visceral cavity through the interspaces of the teeth : if it be actually 

 introduced by the tube-feet it must pass by exosmose through the 

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



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. 



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 cerresponding circle around the anus. A 

 vessel called the intestinal 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 (^fig- 98, y), receiving the veins of the ovaria, sends off 

 five trunks which run in the interspaces of the bases of the tubular 

 feet, or 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 ofiice 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 gene- 

 rally 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 

 will permit of two explanations. In the first and most probable mode 



* CLXVII. p. 85. 



