30 THE MOLLUSC A 



by invagination in some other Gastropods (Fusus), and in Dentalium 

 and various Lamellibranchs. 



Mesoderm and Mesodermic Organs. A third intermediate cellular 

 layer is formed, generally at an early stage, between the external 

 ectodermic envelope and the endodermic wall of the digestive tube. 

 This is the mesoderm, from which all the organs situated between 

 the digestive tube and the integuments are produced. The origin 

 of this layer is often difficult to determine, especially in highly 

 specialised forms, but in all cases in which the origin is distinct 

 there is no doubt about the matter, the mesoderm is derived from 

 the endoderm. This derivation is shown in the Polyplacophora, 

 the Aspidobranchg (Patella, Fig. 1 2, me; Trochus, Neritina\ the Pectini- 

 branchs (Paludina, Bithynia, Crepidida, Fulgur, etc., and seemingly the 

 Heteropoda), the Opisthobranchs (Philine, Umbrella., Aplysia, Clionc, 

 Chronwdoris, etc.), the basommatophorous and stylommatophor- 

 ous Pulmonates, the Scaphopods, the Lamellibranchs (Pisidium, 

 Unionidae, Dreissensia, Teredo, etc.). Nevertheless we find scattered 

 mesodermic cells, giving rise to unicellular muscular fibres of the 

 integument (Unio, Crepidula), which are derived from the ectoderm. 



The principal result of the development of the mesoderm is 

 the formation of another cavity in the embryo, the coelom. In 

 the Mollusca tne coelom does not originate by the invagination 

 of enterocoelic pouches (Tonniges has shown the inaccuracy of 

 Erlanger's description of enterocoelic coelomic pouches in Paludina), 

 but, as a result of specialisation, this primitive method is supplanted 

 by solid mesoblastic masses, generally paired, which may be con- 

 sidered as the cardio-genito-renal rudiments. These mesoblastic 

 masses take their origin from the macromeres. As a rule, at the 

 stage when four macromeres are present, it is the most posterior 

 of the four that gives rise, by successive divisions, to the two first 

 mesomeres or primary mesodermic cells (Fig. 11). From these the two 

 mesodermic bands, which constitute the third layer, are produced 

 as solid, or in some cases discontinuous masses. The coelomic 

 cavity or series of cavities are formed by more or less regular fission 

 or delamination of the mesoblastic bands, evidently a secondarily 

 acquired mode of development. The coelom is therefore physio- 

 logically a schizocoele. Eventually it is placed in communication 

 with the exterior by ectodermic invaginations. The order in 

 which the different parts of the primitive coelomic cavity make 

 their appearance is not constant. The pericardium, in particular, 

 may originate as two symmetrical cavities, which unite more or 

 less rapidly (Paludina, Cyclas, Cephalopoda), or directly, as a single 

 azygos cavity (Dreissensia, Pulmonata). The extension of the 

 mesodermic elements evidently narrows the primitive segmentation 

 cavity or blastocoele, which becomes the cavity of the circulatory 

 system. These elements spread between the ectoderm and endo- 



