VIII 



PHYLUM MOLLUSCOIDA 



361 



the Inarticulata it has no prismatic structure, but usually consists 

 of a chitinoid material more or less strengthened by calcareous 

 spicules, or of alternate chitinoid and calcareous layers. A system 

 of tubules, such as that described above in the case of the example, 

 occurs in many cases. Among the Articulata the loop may be 

 absent ; when present, it varies greatly in form and size, being 

 sometimes very small and simple (Fig. 300, C, D), sometimes bent 

 upon itself, as in Magellania, sometimes attached to the septum or 

 to the interior of the dorsal valve (E), sometimes, as in the extinct 

 Spirifera, represented by a complex double spiral (F), sometimes 

 reduced to short, paired rods springing from the septum (G). The 

 shell is secreted 

 partly by the 

 general surface 

 of the body, 

 partly by the 

 folds of the 

 mantle. 



In some cases 

 spicules occur 

 scattered 

 through the 

 soft parts. 



The majority 

 of both orders 

 are attached 

 by a longer or 

 shorter ped- 

 uncle which 

 passes between 

 the proximal 

 ends of the 

 valves in Lin- 

 gula (Fig. 300, 

 A), through a 



perforation in the ventral valve in Discina (C), and through a 

 foramen in the spout-like posterior end of the ventral valve in the 

 Articulata. Crania (B) has the ventral valve fixed directly to 

 foreign objects, the peduncle being absent. 



The lophophore is found in its simplest form in Cistella 

 (Fig. 301, A), in which it is a horse-shoe-shaped disc with very 

 short arms, attached to the dorsal mantle-lobe, of which it is a 

 prolongation, and surrounded with flexible tentacles which project 

 between the valves. From this the lophophore of Magellania, 

 which may be considered as typical for the Articulata, is easily 

 derived by an increase in size, and by the prolongation of the middle 

 region of the concave edge into a coiled offshoot. In the Inarti- 



FIG. 301. Dissections of A, Cistella ; B, Rhynchonella ; and C, 

 Iiingula. a. anus ; Iph. lophophore ; mth. mouth. (After 

 Schulgin arid Hancock.) 



