BRACHIOPODA. 



313 



the unsegmented end of the body, but by the interpolation of 

 fresh segments at the cephalic end of the body as in Cestodes ; 

 so that the hindermost segment is the oldest. Assuming the 

 correctness 1 of Lacaze Duthiers' observations, the mode of 

 formation of these segments appears to me to render it probable 

 that they are not identical with the segments of a Chaetopod. 

 A suspender is attached to the front end of each embryo. Before 

 the four segments are established the whole embryo is covered 

 with cilia 2 , and two and then four rudimentary eyes are 

 developed on the anterior segment of the body. 



The history of the Larva and the development of the organs of 



tlie Adult. 



Articulata. The observations of Kowalevsky and Morse 

 have given us a fairly complete history of the larval metamor- 

 phosis of 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 Miiller, 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 compart- 

 ments. 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 ap- 

 pearance 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 



FIG. 136. LARVA OF AR- 

 GIOPE. (From Gegenbaur, 

 after Kowalevsky.) 



m. mantle ; b. setas ; d. 

 archenteron. 



1 It should be stated that it is by no means clear from Kowalevsky's figures that 

 he agrees with Lacaze Duthiers as to the succession of the segments. 



2 Kowalevsky in his figures leaves the penultimate lobe unciliated. 



