VIII 



PHYLUM MOLLUSCOIDA 



371 



In this condition the larva swims freely like a trochophore. 

 After a time it comes to rest and fixes itself by its peduncular seg- 

 ment (B). The two lobes of the mantle-fold (m) become reflexed 

 so as to point forwards instead of backwards, thus leaving the 

 peduncular region exposed and covering the head-region : by this 

 process the outer surface of the larval mantle becomes internal, 

 and vice versa. A stomodgeum is 

 formed on the head-region, and, 

 communicating with the mesenteron, 

 establishes the enteric canal. The 

 umbrella-like head-region decreases 

 in size, and perhaps forms the lip, 

 which is at first confined to the 

 part immediately dorsal to the 

 mouth. The lophophore appears at 

 first on the inner surface of the 

 dorsal mantle-lobe, but gradually 

 extends and surrounds the mouth ; 

 in its earlier stages it is circular, but 

 afterwards assumes the horse-shoe 

 form by sending out paired exten- 

 sions. In genera with a complex 

 lophophore, like Magellania, this 

 organ has at first a simple horse- 

 shoe form (Fig. 299, Iph). A shell 

 is secreted by the mantle-lobes, and 

 the peduncular region becomes the 

 peduncle of the adult. 



Distribution. The Brachiopoda 

 are all marine. They are widely 

 distributed geographically, and live 

 at various depths from between 

 tide-marks to 2,900 fathoms. At 

 the present day the class includes 

 only about 20 genera and 100 

 species, but in past times the case 

 was very different. Brachiopods ap- 

 pear first in the lower Cambrian 

 rocks, where the existing genera 



Lingula and Discina are found. No more striking examples can 

 be adduced of persistent types organisms which have existed 

 almost unchanged for the vast period during which the whole of 

 the fossiliferous rocks have been in process of formation. Alto- 

 gether 106 genera are known from the Palaeozoic rocks, 34 

 from the Mesozoic, and 21 in the Cainozoic and Recent periods. 

 Obviously the group is tending, though slowly, towards extinction. 



Researches on fossil and recent forms have shown the 



B B 2 



FIG. 298. Two later stages in the 

 development of Cist ell a. A, 

 free- swimming ; B, after fixation. 

 hs. peduncular region ; in. mantle ; 

 ms. body-region ; md. mesenteron ; 

 ick. ciliated ring ; vs. head-region. 

 (From Lang's Comparative Ana- 

 tomy, after Kowalevsky.) 



