BRACHIOPOUA. 317 



which there is no trace in the young stages, grows out as a simple prolonga- 

 tion of the hinder end of the body while the larva is still free. It had already 

 reached a very great length in the youngest fixed larva observed. 



Development of Organs. 



The alimentary tract after the obliteration of the blastopore 

 forms a closed sack, which becomes subsequently placed in 

 communication with the exterior by the stomodseal invagination. 

 The liver is formed as a pair of dorsal outgrowths of the mesen- 

 teron. From Brooks' observations on Lingula it would appear 

 that the primitive mesenteron forms the stomach of the adult 

 only, and that the intestine grows out from this as a solid 

 process : this eventually meets the skin, and here the anus is 

 formed. In the Articulata the mesenteron is aproctous. 



The origin of the body cavity as paired archenteric diverticula 

 has already been described. Its somatic wall becomes in Lingula 

 ciliated, and its cavity filled with a corpusculated fluid, as in 

 many Cha^topods. It is eventually prolonged into the dorsal 

 and ventral mantle lobes as a pair of horn-like prolongations 

 into each lobe, which communicate with the body cavity by 

 large ciliated openings. Some incomplete observations of Brooks 

 on the development of the nervous system in Lingula shew that 

 it arises in the embryo as a ring round the oesophagus with 

 a ventral sub-cesophageal (fig. 138 q], and two lateral ganglia, 

 and two dorsal otocysts. The ventral ganglion is formed as a 

 thickening of the epiblast, with which it remains in continuity 

 for life. The remainder of the ring grows out from the ventral 

 ganglion as two cords, which gradually meet on the dorsal side 

 of the oesophagus. 



General observations on the Affinity of tlie BracJiiopoda. 



The larva of Argiope, as has been noticed by many observers, has 

 undoubtedly very close affinities with the Chaetopoda. It resembles, in fact, 

 a mesotrochal larval Chcetopod with provisional setae (vide Chapter on 

 Chastopoda). Lacaze Duthiers' observations point to the lobes of the larva 

 not being true segments, and certainly the mesoblast does not in the embryo 

 become segmented as it ought to do were these lobes true segments. If this 

 view is correct the larva is to be compared to an unsegmented Cha^topod 

 larva. In Rhynchonella, however, indications of two segments are afforded 

 in the adult in the two pairs of segmental organs. 



Though the larval Brachiopod resembles a mesotrochal Chastopod larva, 



