392 



MANUAL OF ZOOLOGY. 



of the cavity of the body proper, and sends more or less branched diver- 

 ticula into the pallial lobes " (Huxley). This system of the atrial canals 

 has been looked upon as a rudimentary respiratory apparatus; but its 

 function is more probably to act as an excretory organ, and it certainly 

 serves also to convey away the reproductive elements, the organs for which 

 are developed in various parts of its walls. By Rolleston the pseudo-hearts 

 are looked upon as corresponding with the so-called " organ of Bojanus" 

 of the Lamellibranchiata. 



The function of respiration is probably performed, mainly, 

 if not entirely, by the cirriferous oral arms, as it appears chiefly 

 to be by the homologous tentacular crown of the Polyzoa. A 



Fig. 205. Development of Terelratulina septentrionalis (after Morse). A, Ciliated 

 embryo. B, More advanced embryo, showing commencing segmentation and a rudi- 

 mentary peduncle (/). C, D, E, F, Further stages of the same embryo. G, Advanced 

 embryo, with a very long peduncle (/), and a circular oral crown of cirri (c). H, 

 Interior of dorsal valve, showing the circular crown of cirri, and the intestine (/'). I, 

 Another larva, at the same stage, having the valves opened, and viewed from one 

 side. J, Part of a larva still further advanced, showing the now horse-shoe-shaped 

 crown of cirri ; p Peduncle ; v Ventral valve of shell ; d Dorsal valve ; c Crown of 

 oral ciiri ; i Intestine ; j Setae springing from the edge of the mantle ; / Loop of 

 dorsal valve. (All the figures are highly magnified.) 



unilocular heart is present in some, but apparently not in all, 

 of the Brachiopoda ; and the circulation seems to be mainly 

 carried on through the interstices between the tissues. 



The nervous system consists of a principal ganglion of no 

 great size, placed in the re-entering angle between the gullet 

 and the rectum. In those Brachiopods in which the valves of 

 the shell are united by a hinge, the nervous system attains a 



