364 



ZOOLOGY 



SECT. 



A 



larva swims freely like a trochophore. After a time it comes to 

 rest and fixes itself by the peduncular segment. The mantle-folds 

 become reflexed so as to point forwards instead of backwards, thus 

 leaving the peduncular region exposed and covering the head-region 

 (Fig. 304, B). 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 

 extensions. In genera with a complex 

 lophophore, like Magellania, this organ 

 has at first a simple horse-shoe form 

 (Fig. 305, IpJi). A shell is secreted by 

 the mantle-lobes, and the peduncular 

 region becomes the peduncle of the 

 adult. 



The development of Lingula (Inarti- 

 culata) differs somewhat widely from 

 that of the Articulata. There is no 

 division by definite constrictions into 

 three regions. The head-region does not 

 expand to the same extent : it early 

 develops elevations which are the 

 rudiments of the first-formed tentacles. 

 The ridge from which the mantle-lobes 

 are developed grows forwards from the 

 outset. The coelome is not formed by 

 outgrowth from the archenteron, but 

 appears as a pair of cavities in a pair of 

 masses of mesoderm-cells which are at 



, ]:j rvrnKfr. firms; from thp wall 



UrSt SO11Q 



Q f ^he } a tter : in other WOrds the 



, 



coelome is not lormed as an enteroccele 



Ik 



FIG. 304. Two later stag's in the 

 development of Cistella. A, 

 free-swimming ; B, after fixation. 

 fts. peduncular region ; wi.mantle ; 



ms. body-region ; ?d. mesenterpn ; 



?r/,. ciliated ring ; vs. head -region. , VI 



(From Land's Comparative Ana- DUt as a SCmZOCCeie. 



tomy, after Kowaievsky.) 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 appear 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 for- 



