THE MOLLUSC A 31 



derm, and become specialised to form the internal lining of the 

 circulatory cavity in particular, and may even fill almost entirely 

 the remainder of the blastocoele in the form of a false mesenchyme 

 (caenogenetic or secondary mesenchyme), which constitutes the 

 connective tissue. This naturally restricts the extension of the 

 coelom properly so called, so that it is commonly reduced to the 

 pericardial cavity. Since the mesodermic tissue gives rise, in this 

 manner, to the epithelial wall of the coelomic cavity, to the lining 

 of the circulatory cavity, and to the conjunctive tissue filling up 

 the spaces between the organs, one must recognise it as sharing in 

 the evolution, firstly, of the coelom and the excretory and repro- 

 ductive organs derived from the coelom ; secondly, of the circulatory 

 apparatus the heart, etc. 



The coelom, of which the formation has been described above, 

 is essentially a cavity communicating with the exterior, and its 

 epithelial wall may be differentiated in two special ways into 

 excretory or renal elements, and into reproductive, and therefore 

 caducous elements. In the most primitive process the kidneys are 

 formed in connection with a portion of the coelom, with which 

 they remain in complete continuity (Paludina). In other cases 

 they are formed by a hollowing out of a portion of the mesoderm 

 in contact with the pericardium (Bithynia, Limax, Cyclas, Dreissensia, 

 etc.), or they may be formed independently in their definitive 

 position (Cephalopoda). Eventually each kidney acquires a com- 

 munication with the pericardium, and in all cases makes a connection 

 with the exterior by an ectodermic imagination. The genital 

 organs or gonads originate either from the wall of the coelom or 

 pericardium (Paludina, Dreissensia), or in contact with the coelomic 

 wall (^ '//"/"<), or from a rudiment common to themselves, the 

 pericardium and the kidney, or, finally, from distinct mesodermic 

 elements. The continuity of the pericardium and gonads is 

 well preserved only in the Aplacophora (Fig. 30, C) and adult 

 Cephalopoda (Fig. 252, coe) ; in all other Molluscs the genital organs 

 are separated from the pericardial cavity and acquire communica- 

 tions either with the kidneys or directly with the exterior. In the 

 latter case the terminal portions of the gonaducts, together with 

 the accessory genital glands, are ectodermic in origin. 



The heart may arise from a portion of the wall of the peri- 

 cardium itself (Paludina), or a common rudiment may give rise to 

 the wall of the pericardium and the heart (Pulmonata, Cyclas, 

 Dreissensia, etc.), and in the latter case the origin of the heart may 

 be paired (Cyclas, Cephalopoda) like that of the pericardium itself. 



The larvae of such Molluscs as lay their eggs singly and free in 

 the sea are hatched out very rapidly ; a few hours suffice in the 

 case of Dentalium among the Scaphopoda ; twenty hours in Trochus 

 among the Aspidobranchs ; fourteen hours in Yoldia among the 



