316 ARTICULATA. 



The shell is deposited as to chitinous plates, which subse- 

 quently become calcified. It undergoes in the different genera 

 great changes of form during its growth. 



With reference to the larval stages of other Articulata, a few points may 

 be noted. 



The three-lobed larva of Terebratulina septentrionalis is provided with 

 a special tuft of cilia at the apex of the front lobe. The arms appear to 

 originate, in Terebratulina caput serpentis, as two processes at the sides of 

 the mouth, on which the tentacles are formed. 



Provisional setae do not appear to be formed in the lobed embryos of 

 Thecidium and Terebratulina, but they appear at a later stage at the edge of 

 the mantle in the latter form. The third lobe of Thecidium gives rise to the 

 dorsal and ventral mantle lobes. 



Inarticulata. The youngest stages in the development of 

 the Inarticulata are not known, and in the earliest stages 

 observed the shell is already developed. The young larvae with 

 shells differ however from those of the Articulata in the fact 

 that they are free swimming, and that the peduncle is not 

 developed. 



The larva of Discina radiata has been described by Fritz Miiller (No. 

 331). It resembles generally a larva of the Articulata shortly after the 

 tentacles have become developed. Five pairs of long provisional setae are 

 present, of which all but the hindermost are seated on the ventral lobe of 

 the mantle. Shorter setae are also lodged on the edge of the dorsal lobe. 

 The mouth is placed on the ventral side of a protrusible oral lobe. It is 

 imperfectly surrounded by four pairs of tentacles, which form a swimming 

 apparatus. 



A fuller history of the development of Lingula has been recently supplied 

 by Brooks (No. 325). The youngest larva is enveloped in two nearly similar 

 plate-like valves, covering the two mantle lobes. The mouth is placed at 

 the centre of a disc, attached to the dorsal valve, on the margin of which is 

 a ring of ciliated tentacles. The general position of the disc and its 

 relations may be gathered from fig. 138, which represents a diagrammatic 

 longitudinal vertical section of the embryo. 



With the growth of the embryo the tentacles increase in number, the 

 new pairs being always added between the odd dorsal tentacle and the next 

 pair. There is an axial cavity in the tentacles which, unlike the cavity in 

 the tentacles of the Polyzoa, does not communicate with the perivisceral 

 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 



