262 DEVELOPMENT OF ORGANS. 



cavity. As the tentacles increase in number, the lateral parts of the 

 tentacular disc grow out into the two lateral arms of the adult, while the 

 dorsal margin forms the median coiled arm. These changes are not effected 

 till the larva has become fixed. 



The attachment of the larva was not observed ; but the peduncle, of 

 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 commu- 

 nication with the exterior by the stomodseal invagination. The liver 

 is formed as a pair of dorsal outgrowths of the mesenteron. From 

 Brooks' observations on Lingula it would appear that the primitive 

 mesenteron forms the stomach of the adult only, and that the intes- 

 tine 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 

 Cbatopods. It is eventually prolonged into the dorsal arid 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<?), 

 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 the Brachiopoda. 



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

 undoubtedly very close affinities with the ChaBtopoda. It resembles, in 

 fact, a mesotrochal larval Chsetopod with provisional setae (vide Chapter on 

 Chsetopoda). 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 Chsetopod 

 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 Chsetopod larva, 

 it does not appear to resemble the trochosphere larva? so far described, or 

 the more typical larva} of the Chajtopoda, in that the ring of tentacles, 

 which is probably, as already mentioned, derived from the ciliated ring 



