MOLLUSC A. 



453 



branchiata and Pteropoda, and some Gastropoda (see pp. 116-7). The anus 

 opens into the sub-pallial space. Its typical position is posterior and 

 always above the foot. In one division of Gastropoda, the Anisopleura, 

 where torsion of the visceral dome has taken place, it is either on the 

 right side or anterior. It has the same anterior position, either on the 

 right or left side, in Pteropoda. 



All Mollusca are furnished with a heart except the Scaphopoda. This 

 heart consists of a thick-walled ventricle giving off an aorta single or 

 double, and receiving blood from a single (most Gastropoda, Pteropoda) 

 or double (i. e. right and left) auricle, in both cases obviously derived from 

 a special dilatation and thickening of the branchial veins bringing back 

 the blood from the ctenidia. But in Nautilus (Cephalopoda Tetrabranchiatd}, 

 the only Molluscan with four gills, and therefore four efferent branchial 

 veins, the four auricular dilatations are not well marked, and scarcely if 

 at all thickened. The arterial system of vessels is very well developed, 

 and capillaries are present in many Cephalopoda and some Pulmonate 

 Gastropoda, but as a rule the blood-system is to a great extent lacunar. 

 Sinuses or veins with defined walls usually convey the blood to the 

 ctenidia. Amoeboid corpuscles are always present. Haemoglobin is very 

 rarely found. Haemocyanin a copper-containing pigment, blue when 

 oxydised, colourless when deoxidised, and acting as an oxygen-carrier, has 

 been detected in various Mollusca. The heart is lodged in a pericardium 

 or secondary coelome. It is apparently a closed off portion of the coelome 

 which is not filled with blood and communicates with the exterior through 

 the nephridia, in Nautilus alone by independent openings. The rest of 

 the coelome has the form of irregular spaces filled with blood and lodging 

 the viscera. The entrance of water into the blood-system, either through 

 special pores, inter-epidermic channels, or through the nephridia and the 

 pericardium, appears to be very questionable. A ' pericardial gland ' 

 which appears to have an excretory function is present in Cephalopoda, 

 some Gastropoda and various Lamellibranchiata. 



There are four nephridia or renal sacs in Nautilus, two in other 

 Mollusca, with the exception of the Pteropoda and the majority of the 

 Gastropoda Anisopleura, in which one sac is aborted. They communicate 

 internally with the pericardium and open externally near the anus, except 

 in Lamellibranchiata where the apertures are anterior. The nephridial 

 sac is often differentiated into a non-secreting duct, and a glandular 

 portion which is frequently dark-coloured from the presence of excretory 

 products in the epithelium. It is ciliated internally except in Cephalopoda, 

 and the ciliary currents appear to set outwards 1 . 



1 Cells of a whitish colour and containing uric acid are found in lines between the coils of the 

 intestine in Cydostoma elegans, forming the ' Concrement-driise ;' Barfurth, Z. A. vii. 1884. Cells 

 either pigmented or not pigmented, and containing uric acid are found scattered as a 'diffuse 



