NATURAI. HISTORY OF AMERICAN LOBSTER. 243 



red blood becomes deeper in color as it takes up oxygen. The bluish tint of the larval 

 lobster is probably due in part to the hamocyanin of its blood. The blood is also 

 regarded as the bearer of other pigments, the lipochromogens, which are probably 

 elaborated in the digestive gland, transmitted by the blood, and laid down in the pig- 

 ment cells and the shell. 



The heart begins to pulsate rythmically when the lobster is an embryo, between 

 4 and 5 weeks old," at a time when the black pigment spots of the compound eyes have 

 begun to show, but when the nervous system has been only roughly blocked out and 

 long before any nerves are developed. The heart, although later brought under nervous 

 subjection and control, is at first quite automatic and independent in its movements. 



The circulatory system of the lobster (see pi. xxxiii) consists (i) of a muscular heart 

 for driving the blood, (2) of arteries or definite channels for conveying it to the tissues, 

 and (3) a system of irregular channels called sinuses or lacunae, besides certain well 

 defined vessels, the veins for leading it back to the pericardial chamber and heart. 

 The arteries end in microscopic capillaries which open directly into the lacunar system. 



The freshly aerated blood of the lobster is driven from the gills to the pericardial 

 sinus, enters the heart through the ostia, is pumped thence by the rhythmical contrac- 

 tions of its walls into the arteries, and by their subdivisions is distributed over the 

 entire body. Having performed its physiological work of giving up to the tissue cells 

 dissolved oxygen and food materials, and having received from them carbon dioxide 

 and other waste products, it returns by the lacunar system to the large ventral sinus, 

 which surrounds the ventral nerve-chain; thence the venous blood is driven to the gills, 

 where aeration is effected by the absorption of oxygen from the fresh streams of sea 

 water in which they are constantly bathed. More simply expressed, the path traversed 

 is heart, body, gills, heart. The gills are placed in the returning blood stream, so 

 that the vessels which both supply the gills with venous blood (afferent branchial ves- 

 sels) and which conduct arterial blood from the gills to the heart (efferent branchial 

 and branchio-cardiac vessels) may be described as veins. 



THE HEART. 



Examining the heart more closely, it appears as a boat-shaped or somewhat hexag- 

 onal body, rounded below, flattened above, and broader in front. It is pierced 

 by three pairs of openings, the dorsal, ventral, and lateral ostia, which admit blood 

 from the pericardial sinus. Each ostium is provided with valves which open inward, 

 so that the blood once admitted to the heart can not be regurgitated to the sinus. 



The heart gives off a series of arteries, five in front and two behind; these are also 

 supplied with valves (or at least in the largest of them, the sternal), so that the heart 

 can empty only into the arteries, while it can fill only from the sinus. 



THE PERICARDIAI. SINUS. 



The chamber in which the heart is suspended, called the pericardial sinus, lies at 

 the extreme upper and hinder part of the carapace; it is lined with connective tissue 



o The beating of the embryo lobster's heart has been noted in winter (December 14) at 100 times per minute. 

 62399°— II 7 



