XII 



BKACHIOPODA 



415 



coe.c 



segment is absent and attachment is effected by the whole surface 

 of one valve. 



It is obvious, therefore, that the foot segment cannot be regarded 

 as a true segment in the Annelidan sense of the word. It is 

 probably to be regarded as a ventral protrusion of the body, similar 

 to the protrusion which forms the greater part of the body in 

 Podaxonia, with this difference, that in recent Brachiopoda, at any 

 rate, it remains solid and no part of the alimentary canal passes into it. 



But we have already reached the conclusion that the Podaxonia 

 represent an early offshoot from the 

 Annelidan stem, and, if the Brachio- 

 poda are to be regarded as showing 

 affinities with the Podaxonia, Morse's 

 general conclusion would be justified 

 in a roundabout manner. 



Conklin's researches have added 

 certain points in favour of Morse's 

 view. The existence of an apical 

 plate with a tuft of long cilia, and 

 above all the position of the mouth 

 as ventral and posterior to the top- 

 shaped ciliated head segment, which 

 may be regarded as representing as 

 it does in Podaxonia an enlarged 

 prototroch, are facts which seem to 

 point to an essential identity of 

 structure between the Brachiopod 

 and Trochophore larvae. 



On the other hand, the origin 

 of the coelom as a division of the 

 archenteron, and the indeterminate 

 cleavage, are points in which the 

 development of the Brachiopod 

 diverges widely from that of any 

 Trochophore larva so far studied, 

 unless Erlanger's results with regard 



to the development of Paludina should be finally sustained. They 

 recall forcibly the development of Echinodermata. 



We shall reserve a final conclusion on the affinities of the 

 Echinodermata and Brachiopoda until the development of the former 

 group has been studied, but we may remark, in the meantime, that 

 the coelom might be described as a hollow, bilobed but unpaired, 

 ventral diver ticulum of the gut. 



If we view it in this light we shall be forcibly reminded of 

 Erlanger's still unrefuted statement as to the origin of the coelom in 

 Paludina. We had already arrived at the conclusion that, in the 

 Trochophore, the origin of the coelomic mesoderm from a single large 

 mother cell (4d), which was originally situated in the gut wall, 



v.v. 



FIG. 329. Youug Terebratulina septen- 

 trionalis some little time after 

 metamorphosis, viewed from the 

 dorsal surface. (After Morse.) 



art, articulating processes of valves a 

 notch on each side on dorsal valve ; coe.c, 

 blind extensions of mantle-coelom perforat- 

 ing the shell ; d.v, dorsal valve ; m, mouth ; 

 oes, oesophagus ; peel, stalk ; st, stomach ; 

 ten, tentacles of lophophore ; v.v, ventral 

 valve. 



