278 INVERTEBRATE MORPHOLOGY. 



nective tissue, but more usually it is reduced to a system of 

 lacunar spaces (a so-called schizoccel) by the development of 

 muscle-bundles traversing it in various directions. A special 

 portion of it (the so-called enteroccel) is, however, always en- 

 closed in definite walls lined by a peritoneal layer of cells, 

 forming a cavity, the pericardium (Fig. 122, s), which lies nor- 

 mally near the dorsal surface of the body, containing the 

 heart and having the inner ends of the nephridia (m) opening 

 into it. The blood vascular system consists of a primitively 

 three-chambered heart (Fig. 122, v) enclosed within the 

 pericardium and composed of a tubular muscular ventricle 

 and two wing-like auricles which open into the ventricle, 

 their openings being guarded by valves which prevent regur- 

 gitation. From the anterior and posterior extremities of the 

 ventricle aortse arise, which, however, as a rule soon lose 

 themselves in the ccelomic lacunae. There is thus no distinc- 

 tion between the blood and pseud-hsemal fluids in the Mol- 

 lusca, since the blood vascular system is not closed. The 

 blood is a colorless fluid in which numerous amoeboid cells 

 float and which holds in solution a substance, haemocyanin, 

 which subserves a respiratory function in a manner similar 

 to the haemoglobin of the Vertebrata. 



The heart is a systemic heart, as is usual in the Invertebrata, and con- 

 titins only aerated blood, which it propels through the lacunas of the body. 

 Keturning from these, the blood passes either directly to the respiratory 

 organs or branchiae, or else a greater or less portion of it traverses first the 

 walls of the nephridia and then passes to the branchias. From these, in 

 which it is aerated, it is received into the auricles, and on their contraction 

 is forced into the ventricle. 



In some MoUusca respiration is carried on by the general 

 surface of the body, but such an arrangement must be re- 

 garded as the exception. As a rule special respiratory or- 

 gans are present in the form of one or more pairs of plume- 

 liks processes {ctenidia) of the body-wall (Fig. 122, i) lying 

 free in the mantle-cavity. They have various forms in the 

 different groups, but consist essentially of a central axis con- 

 taining an afferent and efferent canal for the blood and bear- 

 ing a single or double series of filaments whose walls are thin 

 and whose ectoderm is ciliated, an interchange of the gases 



