696 SPECIAL PHYSIOLOGY. 



into lacunse, or interstices, between the organs or elements of the tissues, 

 from which it is collected into venous sinuses and trunks. 



Mollusca. The most perfect condition of the circulatory system, in the 

 Non-vertebrate animals, is met with in this Subkingdom. and in the Class 

 Cephalopods. In the cuttle-fish, for example, there is found a systemic ven- 

 tricular heart, provided with valves at its orifice ; it is usually rounded, has 

 strong muscular walls, and even internal columnar carnese. Arteries proceed 

 from it to all parts of the body, excepting to the branchia? or gills, the liver 

 even receiving branches. The blood is returned into a large vein, or venous 

 sinus, which is surrounded by a remarkable cellular organ filled with blood, 

 and from which symmetrical lateral branches, two or four in number, accord- 

 ing to the number of the gills, proceed to those organs, each presenting, as it 

 enters the gill, a pulsating dilatation, or so-called branchial heart, which helps 

 to propel the blood through the gills. From the gills, the blood is returned 

 into large venous sinuses, which, being contractile, act as auricles, and thence 

 passes into the ventricular systemic heart already described. In the Ptero- 

 pods and in the Gasteropods, there is but a single heart, which 'is always sys- 

 temic, distributing its contents by one arterial trunk and numerous branches, 

 to the body and liver, from which, having passed through lacunce or spaces, it 

 is again collected by veins, and by them conveyed to the respiratory organs ; 

 from these, it is collected by other canals, the branchio-cardiac veins, and is 

 so brought to the- heart again. In the terrestrial Pulmogasteropods, as in 

 Helix, the venous blood from the body passes through small vessels on the 

 walls of the pulmonary air sac, and is then collected into a larger vessel, 

 which conveys it to the heart ; whilst in the aquatic Branchiogasteropods, as 

 in Doris, the blood returning from the body is carried, by special vessels, into 

 the gills, and is then conveyed by other vessels back to the heart. In both 

 kinds of Gasteropods, the heart consists of an auricle and ventricle, between 

 which there is found a distinct but minute valve, which serves accurately to 

 direct the course of the circulating fluid. In the Lamellibranchiata, the heart, 

 usually single, but sometimes double, in correspondence with the bilateral 

 arrangement of the parts of the body of these animals, and often perforated 

 by the intestine, is placed in a pericardium, situated near the adductor mus- 

 cle, which closes the shell ; when single, it has sometimes one and sometimes 

 two auricles, connected with its simple ventricle ; when the heart is double, 

 each has only one auricle. 



Molluscoida. In these animals, the circulatory organs become still more 

 simple, or even disappear. Thus, in the Ascidioida or Tunicata, although the 

 bloodvessels, or blood sinuses, are very complex on the walls of the peculiar 

 respiratory atrium, the heart itself has no valves between its dilated parts or 

 chambers, and the blood is propelled by opposite peristaltic actions, separated 

 by an interval or pause, first in one direction and then in the other ; hence the 

 heart is sometimes systemic and sometimes respiratory, and each chamber is 

 sometimes auricular and sometimes ventricular in function. In the compound 

 Ascidioida, it is stated that the vessels of one animal are connected, through 

 the footstalk or common "stolon," with those of the others belonging to the 

 same cluster. In the Brachiopoda, there is still a proper simple heart, some- 

 times, it is said, two ; but the circulating system is considerably reduced in 

 extent: the numerous contractile cavities found in these animals, have not 

 been proved to have any connection with the blood system, but rather to be 

 comparable with the atrium of the Tunicata. Lastly, in the Polyzoa, neither 

 a contractile heart, nor even vessels, have yet been detected ; in these soft deli- 

 cate animals, the nutritive fluids are supposed to permeate the walls of the 

 alimentary canal into the perivisceral spaces, and thence to penetrate every 

 part of the body. 



Annulosa. In the largest animals of this Subkingdom, the circulatory ap- 

 paratus presents the greatest simplification. Thus, amongst the Crustaceans, 

 a single, well-developed, muscular, systemic, dorsal heart is found in the lob- 

 ster ; it isj)laced in the median line, beneath the hinder border of the thoracic 

 part of the shell, and is inclosed in a delicate sac, which, from its resemblance 

 to the pericardium in the Vertebrata, has been so named, but which is, in 

 reality, a venous sinus. From the heart, six systemic arteries are given off to 



