MORPHOLOGY OF MELIBE 551 



Opisthobraiichiata; Lankester (1893) on gasteropods 

 and other molluscs ; Goodrich (1895) on nematodes, chitons, 

 Peripatus, &c. ; Hecht (1895) on nudibranchs ; Lang 

 (1896) on the Mollusca, (1900) nudibranchs; Shipley and 

 Macbride (1915) on the molluscs : and others, have thrown 

 a great deal of light on the nature of the organs of circulation 

 in Invertebrata, particularly the works of Sedgwick 

 (1888), Lankester (1893), Goodrich (1895), and Lang (1900). 



(1) The Pericardium. 



According to Hancock (1864 : 513) the so-called pericardium 

 in Dor ids lies immediately above the renal chamber and 

 directly below the dorsal skin in front of the branchial circle. 

 It is, with the exception of the opening leading into the 

 pyriform vesicle, a closed membranous sac, formed apparently 

 by what has been designated the peritoneum, and is just 

 sufficiently large for the accommodation of the dilated auricle 

 and ventricle. It is lined with its own proper membrane 

 which is closely adherent to and intimately confounded with 

 the peritoneal membrane, but can be observed reflected upon 

 the heart at the root of the aorta. It has just been stated that 

 this cavity is closed — previous communications to the contrary 

 were erroneous owing to defective material used. 



Goodrich (1895 : 484-6) maintains that the vascular system 

 or blood system is simply a liquefaction, as it were, of the 

 mesoblast (Lankester's view). This corresponds with facts 

 as found in Diploblastica, e.g. (adult) Coelenterata, 

 where blood-spaces are entirely absent, ' . . . while as to the 

 nematodes, ... it seems probable the body-cavity is a blood- 

 space, corresponding in relation to the parenchyma of the 

 planarians/ He shows, following Erlanger, how the peri- 

 cardium in Paludina arises as two coelomic sacs on either 

 side — a hollowing out of the mesoblast. These coelomic 

 cavities then fuse, and later by processes of special growth 

 form a peritoneal funnel that opens to the outside on either 

 side. ' The gonad develops from the wall of the coelom ; 

 then together with the rudimentary left peritoneal funnel, it 



