128 



In reviewing, for a moment, the disposition of the organs of circulation in those 

 classes of invertebrate animals we have noticed, we shall find the confirmation of 

 a law in the formation of the internal organs of animals which has been noticed 

 and promulgated by Serres, Geoffrey-St.-Hilaire, and Meckel ; viz., that the vari- 

 ous degrees of development which an animal, high in the scale of being, passes 

 through from the first moment of conception to a period of full maturity, corres- 

 pond to the permanent states of development in the lower grades of the animal 

 series. Thus, in relation to the vascular system, we find at first but one system of 

 vessels in the embryo. This condition of the vascular system resembles the per- 

 fect state of these organs in the medusae and other zoophytes which have but one 

 system of vessels ; and the resemblance is the more striking, since in both in- 

 stances, the vessels are not distinct from the general mass of the body. At a 

 more advanced stage of development, the central organ of the circulation presents 

 a mere dilated oblong canal, hardly possessing muscularity. In this stage we have 

 the analogy with the greater part of the annelidae, or red-blooded worms, where 

 the heart is a mere dilated tube. In the arachnidae and some Crustacea;, the heart 

 is a thin elongated sac, from the extremities of which the blood-vessels arise. In 

 the primitive state of the development of the heart in higher animals, there exists 

 but one dilatation, as in the arachnidae and Crustacea?, when perfect. In a sub- 

 sequent degree of development, where a second dilatation is produced by the sepa- 

 ration of the auricles, or receiving cavities of the heart, from the general system of 

 veins returning the blood, we have the analogy of the embryo state of the higher 

 animals with the perfect formation of the mollusca, fish, and the lowest orders of 

 reptiles. 



We shall trace the remaining analogies between these states, when speaking, 

 in a subsequent paper, of the circulation in vertebrate animals. At present we 

 have followed it as far as the invertebrate classes will permit us. 



LEECHES. 



It is stated by Mr. Gay, in a letter from Chili, that Leeches there inhabit the 

 woods, and never are found in water. He has frequently had his legs wounded by 

 them in traversing the country. Only one aquatic species is known to him at 

 Valdivia, and one at Santiago. Another interesting fact indicated by him, is the 

 tendency of reptiles in these southern regions to become viviparous ; an anomaly 

 which Mr. Gay has observed in a great number of Ophidians, Iguanas, and even 

 in one species of Frog. 



