CHAPTER XL 



BRACHIOPODA 1 . 



THE observations which have been made on the developmental 

 history of the Brachiopocla have thrown very considerable light on the 

 systematic position of this somewhat isolated group. 



Development of the Layers. 



For our knowledge of the early stages in the development of the 

 Brachiopoda we are almost entirely indebted to Kowalevsky * (No. 326). 

 His researches extend to four forms, Argiope, Terebratula, Terebratu- 

 lina, and Thecidium. The early development of the first three of 

 these takes place on one plan, and that of Thecidium on a second plan. 



In Argiope, which may be taken as typical of the first group, the 

 ova are transported into the oviducts (segmental organs) where they 

 undergo their early development. The segmentation leads to the 

 formation of a blastosphere, which then becomes a gastrula by invagi- 

 nation. The blastopore gradually narrows, and finally closes, while 

 at the same time the archenteric cavity (fig. 135 A) becomes 

 divided into three lobes, a median (me) and two lateral (pv~). These 

 lobes next become completely separated, and the middle one forms 

 the mesenteron, while the two lateral ones give rise to the body 

 cavity, their outer walls forming the somatic mesoblast, and their 

 inner the splanchnic (fig. 135 B). The embryo now elongates, and 

 becomes divided into three successive segments (fig. 135 B), which are 

 usually, though on insufficient grounds (vide Thecidium), regarded as 



1 The classification of the Brachiopoda adopted in the present chapter is shewn in 

 the subjoined table: 



I. Articulata. { ?S U " 1 < " ? ' 



(a. Lingulidffi. 



II. Inarticulata. ]& Craniadas. 

 ( c. Disciuidae. 



2 Kowalevsky's Memoir is unfortunately written in Eussian. The account in the text 

 is derived from an inspection of his figures, and from an abstract in Hoffmann and 

 Schwalbe's JahresbericMe for 1873. 



B. E. 17 



