30 THE MOLLUSCA 
by invagination in some other Gastropods (/'usus), and in Dentaliwm 
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 Aspidobranchs (Patella, Fig. 12, me; Trochus, Neritina), the Pectini- 
branchs (Paludina, Bithynia, Crepidula, Fulgur, etc., and seemingly the 
Heteropoda), the Opisthobranchs (Philine, Umbrella, Aplysia, Clione, 
Chromodoris, etc.), the basommatophorous and stylommatophor- 
ous Pulmonates, the Scaphopods, the Lamellibranchs (Piszdium, 
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 the coelom does not originate by the invagination 
of enterocoelic pouches (Ténniges 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 (Puludina, 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- 
