342 



ZOOLOGY 



SECT. 



ing four groups of provisional setae. Soon this mantle-fold divides 

 into dorsal and ventral lobes, which, being directed backwards, 

 cover the peduncular region. 



In this condition the larva swims freely like a trochosphere. 

 After a time it comes to rest arid 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 back- 

 wards, 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 

 stomodaeum 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 region immediately dorsal to the 

 mouth. The lophophore appears at 

 first on the inner surface of the 

 dorsal mantle-lobe, but gradually ex- 

 tends and surrounds the mouth. In 

 its early stages it is circular, but 

 afterwards assumes the horseshoe 

 form by sending out paired exten- 

 sions. In genera, like Magellania. 

 with a complex lophophore, this 

 organ has at first a simple horse- 

 shoe form (Fig. 274, Ipli.). A shell is 

 secreted by the mantle-lobes, and the 

 peduncular region becomes the pe- 

 duncle 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 twenty 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 fossi lite roiis rocks have been in process of formation. Alto- 

 l(M) genera are known from the palaeozoic rocks, thirty-four 



rad 



FK,. [273. Two later stages in the 

 development of Cistella. A, 

 free-swimming; B, after fixation. 

 Its. peduncular region ; m. mantle 

 ms. body-region ; nut. mesenteron 

 /'/. ciliated ring ; r*. head-region 

 (From Lang's Compnratire Anat 

 num. after Kowalevsky.) 



