FOEM OF BODY OF BEACHIOPODA. 



307 



Bibliography. 



ChvEy, R., On the anatomy of the Brachiopoda. Transact. Zoolog. Soe. Vol. I. 1835.— "Vogt, C, 

 Anatomie der Lingula anatina. Denkschr. der schweiz. Gesellsch. fiir die gesammte Naturwis- 

 senseh. Bd. VII. 1812.— Huxley, Ann. and Mag. Nat. Hist. 1854.— Gratiolet, Journal de Con- 

 chiliologie, 1857-60.— Hancock, A., Phil. Transact. 1858.— Lacaze-Duthieks, Sur la Thecidie. 

 Ann. so. nat. IV. xv.— Moese, B., On the systematic position of the Brachiopoda. Proceed, of 

 Boston Soc. of Nat. Hist. Vol. XV.— The same, Embryology of Terebratulina. Mem. of Bost. 

 Soc. Vol. II.— Kowaxevskt, Bcohacht. liber die Bntwickelung der Brachiopoden, Moskan, 1874, 

 (Russian.) 



Form of the Body. 

 § 239. 



It is necessary to go back to embryonic stages to understand 

 the characteristics which distinguish the form of the body in the 

 Brachiopoda. In it, and at an early period, we meet with a stage 

 in which the previously undivided body is separated into three (in 

 Thecidium four) metameres ; this discloses the Annulate type. A 

 terminal circlet of cilia is predominant among the cilia formed all over 

 the body in Terebratula, as is the case also in many Annelid larvae. 

 Bundles of setae (Fig. 163, d) appear on the middle segment, which 

 can be moved just as in the Chaetopoda, 

 while the first metamere (cephalic segment) 

 is converted into an umbrella-like enlarge- 

 ment over the mouth ; this is surrounded 

 by long cilia (Argiope). In this point also 

 we can make out an affinity with Vermian 

 larvae (Actinotrocha). 



While the larva becomes attached by the 



last metamere, two elevations are formed 



from the middle one, which enclose the first, 



and take on the form of two mantle-folds. 



The two shells, which can be distinguished 



as dorsal and ventral, are developed from 



this, and reach to the stalk formed from the 



last metamere. Owing to their position on 



the body, the shells are clearly quite unlike 



those of the Mollusca ; this formation of the 



shells is a peculiarity, which marks off the 



Brachiopoda. And further this formation 



is probably the real cause of the cessation 



of the development of metameres, which 



is also connected with the fixed position of the animal. Another 



peculiarity, the development of arms, is explained by the mode 



of life. 



x 2 



Fig. 163. Larva of Argiope. 



m Mantle. b Bundles of 



seta^. d Enteron (after 



Kowalevsky) . 



