LOLIGO PEALII LESUEUR 239 



glands. The hindgut or proctodeum is represented by the rec- 

 tum. Connected with the hindgut and embryologically derived 

 from it as an invagination of the distal end of the proctodeum is 

 an organ situated above the rectum and called the ink-sac. It is 

 shaped like an elongated gourd and its duct opens into the anal 

 chamber. The glandular portion of the enlarged end of the ink- 

 sac produces a black liquid which can be squirted through the 

 anus and out of the siphon, thus hiding the squid from the pur- 

 suit of an enemy. The iridescent color of the walls of the 

 ink-sac is due to iridiocysts. 



Circulatory system. The circulatory system of the squid 

 is a closed one. There are three central pulsating organs, one 

 of which is arterial and is called the systemic heart, while the 

 other two are venous and are called the branchial hearts. The 

 systemic heart is situated between the two gills at their base, 

 below the stomach. It consists of a single chamber or ventricle. 

 In the European squid, Saepia officinalis, the branchial veins 

 are dilated at the end, thus forming two auricles. But in the 

 American common squid auricles are absent. The systemic 

 heart receives only oxidized blood through two afferent vessels 

 the right and left branchial veins and distributes it through 

 three efferent vessels the anterior aorta, the posterior aorta, 

 and the genital artery. The openings of the branchial veins 

 into the systemic heart are protected each by a pair of semi- 

 lunar valves preventing the return of the blood to the gill. The 

 openings of the anterior and posterior aortas are protected each 

 by a single semilunar valve preventing the return of the blood to 

 the heart. The anterior aorta arises from the anterior projection 

 of the heart and runs forward to the head in close contact with 

 the oesophagus. In the head it gives off branches and communi- 

 cates through capillaries with the anterior vena cava. The 

 posterior aorta arises from the posterior projection of the heart 

 and soon splits into three pallial or mantle arteries. Of these the 

 median mantle artery runs forward along the median line of the 

 lower surface of the mantle, while the two lateral mantle arteries 



