RR AC 1110 POD A. 



2G1 



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 Terebratuliua septentrional is is provided with 

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

 originate, in Terebratulina caput serpeiitis, as two processes at the sides 

 of the mouth, on which the tentacles are formed. 



Provisional setee 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. 



One of these larvae has been described by 

 Fritz Miiller (No. 331), and is very probably that 

 of a Crania. 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 hinder- 

 most 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 Lin- 

 gula 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. 



O * ' 



The general position of the disc and its relations 

 may be gathered from fig. 138, which represents 

 a diagrammatic longitudinal vertical section of 



o o 



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 



FIG. 138. DIAGRAM OF A 

 LONGITUDINAL VERTICAL SEC- 

 TION OF AN ADVANCED EM- 

 BRYO OF LINGULA. (After 

 Brooks.) 



a. end of valves ; b. thick- 

 ened margin of mantle; c. 

 mantle; d. dorsal median 

 tentacle; e. lophophore; /. 

 lip; g. mouth; h. mantle 

 cavity; i. body cavity; k. 

 wall of O3sopkagus ; I. ceso- 

 phagiis; m. hepatic cham- 

 ber of stomach ; n. intesti- 

 nal chamber of stomach ; o. 

 intestine; q. ventral gang- 

 lion ; r. posterior muscle ; 

 s. dorsal valve of shell; t. 

 ventral valve of shell. 



