BRACHIOPODA. 



259 



some of the Articulata, while some of the later larval stages in the 

 history of the Inarticulata have been made known to us from the 

 researches of Fritz Muller, Brooks, etc. The embryo of Argiope, 

 which may be taken as the type for the Articulata, was left (fig. 

 135 B) as a three-lobed organism with a closed mesenteron and a 

 body cavity divided into two lateral compartments. On the middle 

 segment of the body dorsal and ventral folds, destined to form the 

 mantle lobes, make their appearance, and on the latter two pairs of 

 bundles of setae are present (fig. 135 B). The setae together with 

 the mantle folds grow greatly, and the setae resemble in appearance 

 the provisional setae of many Chaetopods (fig. 152). On the hinder 

 border of the mantle cilia make their appearance. The anterior or 

 cephalic segment assumes a somewhat umbrella-like form, and round 

 its edge is a circlet of long cilia, while elsewhere it is provided with 

 a coating of short cilia. Two pairs of eyes 

 also arise on its anterior surface (fig. 136). 



After swimming about for some time the 

 larva becomes fixed by its hind lobe, and 

 becomes gradually transformed into the adult. 

 The hind lobe itself becomes the peduncle. 

 After attachment the mantle lobes bend 

 forward (fig. 137 A, in), and enclose the ce- 

 phalic lobe. The valves of the shell are 

 formed on their outer surface as two delicate 

 chitinous plates (fig. 137 B). At a somewhat 

 later stage the provisional bristles are thrown 

 off, and are eventually replaced by permanent 

 setae round the edge of the mantle. The 

 cephalic lobe becomes located in the dorsal 

 valve of the shell, and the mouth is formed 

 near the apex of the cephalic lobe imme- 

 diately ventral to the eye-spots, by an epi- 

 blastic invagination. The permanent muscles 

 are formed out of the muscles already present 

 in the embryo. 



Around the mouth there arises a ring of 



tentacles, very possibly derived from the ciliated ring visible in 

 fig. 136 1 . The ring of tentacles is placed obliquely, and the mouth is 

 situated near its ventral side. The tentacles appear to form a post- 

 oral circlet, like that of Phoronis (Actinotrocha) : they gradually in- 

 crease in number as the larva grows older. 



Some of the later stages hi the development of the Terebratulidse have 



1 In the abstract in Hoffman and Scbwalbe Kowalevsky is made to state that the 

 tentacles spring from the border of the mantle. This can hardly be a correct account 

 of what he states, since it does not fit in with the adult anatomy of the parts. The 

 figures he gives might lead to the supposition that they sprang from the edge of the 

 cephalic lobe, or perhaps from the dorsal lobe of the mantle. 



172 



FIG. 136. LAKVA OF AR- 

 GIOPE. (From Gegenbaur, 

 after Kowalevsky.) 



m. mantle ; b. setse ; d. 

 archenteron. 



