67 



of the hearts are directly bathed by the blood. My most careful preparations 

 have failed to demonstrate an endothelium on the inner surface of the branchial 

 and systemic hearts yet both have intrinsic arteries and veins which are 

 probably connected with capillaries. Marceatj finds that the walls of the hearts 

 are either covered by an endothelium or by glandular epithelium. The presence 

 of capillaries in the Dibranchs has been recognized for a long time but it has 

 not been known whether lacunae — i. e. primary body cavities — exist. The 

 accepted belief has been that they are present and are represented in part by 

 the sinuses of the head ; but since capillaries are found in every part of the 

 squid and lacunae in none we must believe that the lacunae do not exist. The 

 sinuses cited as examples of lacunae are certainly bounded by an endothelium. 



The Coelom. The coelom or secondary body cavity is formed by the union 

 of the dorsal portion of the pair of tubular cavities whose ventral portion forms 

 the nephridial sac (compare p. 39). The coelom occupies the dorsal half of the 

 visceral mass and contains the gonad , the stomach , caecum , intestine , and the 

 systemic and the branchial hearts. It communicates with the exterior through 

 the ciliated tubes which lead to the nephridial sac and it is lined by a ciliated 

 pavement epithelium , the peritoneum , which is reflected over the organs within 

 the cavity. The systemic heart is attached to the lower wall of the coelom by a 

 mesentery which is fastened along the median line of its lower surface and also 

 by the peritoneum reflected over the two branchial veins and the large aortae as 

 they enter the heart. The branchial hearts are enclosed in coelomic evaginations , 

 the branchial pericardia , which open into the coelom through narrow oblique slits 

 near the dorsal end of the systemic heart. The genital products are shed into 

 the coelom , from which they are removed by the ciliated funnels of the vas 

 deferens or the oviduct. There is apparently nothing to prevent the escape of 

 the genital products from the coelom through the nephridium unless the cilia of 

 the peritoneum drive the ova and sperm toward the opening of the sexual duct. 

 The coelom is not divided , as in some Dibranchs , into a dorsal genital chamber 

 and a ventral pericardium which are united by a narrow tube. The coelom nor- 

 mally holds a small amount of fluid which contains a few amoeboid cells. It is 

 commonly infested by Cestodes which float in a large amount of fluid. 



