272 INVERTEBRATE MORPHOLOGY. 



project into the canals of the shell have been stated to be 

 sensory, containing an axial nerve-fibre terminating in a sen- 

 sory cell. 



The nephridia (Fig. 120, ne) are represented by two or 

 four (Rhynchonella) funnel-shaped short tubes which open by 

 a fimbriated mouth at one extremity into the coelomic cavity 

 and, rapidly narrowing towards the outer end, open by a small 

 pore into the mantle-cavity. In addition to their probable 

 excretory function, these structures, as in some of the Anne- 

 lida, serve also as ducts for the passage to the exterior of the 

 reproductive elements. These are derived from the coelomic 

 peritoneum and form branching masses (Fig. 120, ov) lying in 

 some cases in the coalomic spaces of the mantle, or in addition 

 extending into the body, as in most Ecardiues, or, as in Tere- 

 bratulina, confined to this region. Most of the Brachiopods 

 are bisexual apparently, though it is possible that Lingula 

 and perhaps some other forms may be hermaphroditic, the 

 male and female elements maturing at different times. 



Development and Affinities of the Brachiopods. The Testi- 

 cardines are characterized by the occurrence of a free larval 

 stage destitute of a shell. In Argiope (Fig. 121) it is appar- 

 ently divided into four segments, the most 

 anterior of which bears two eye-spots and 

 assumes an umbrella-like form, long cilia 

 projecting from the margin. The third 

 segment develops two folds which enclose 

 the posterior segment and bear on their 

 margin bunches of setae inserted in seta- 

 sacs and recalling the setae of certain Annelid 

 larvse. After swimming about for a time 

 the larva settles down and fastens itself by 

 the posterior segment and the mantle-lobes 

 FIG. 121. --LARVA turn forward to enclose the anterior seg- 



OF Argiope (after ml 



KOWALEWSKI). ments. 1 he posterior segment becomes the 



peduncle of the adult, and the shell de- 

 velops on the surface of the mantle-lobes, whose bunches of 

 setae are thrown off. The mouth makes its appearance only 

 after fixation just ventral to the eye-spots, and around it there 

 develops a ring of tentacles placed somewhat obliquely, and 



