MOLLUSCOIDA: BRACHIOPODA. 273 



Within the pallial lobes there is a remarkable system of 

 more or less branched tubes, anastomosing with one another, 

 and ending in caecal extremities. This, which has been 

 termed by Huxley the "atrial system," communicates with the 

 peri visceral cavity by means of two or four organs which are 

 called " pseudo-hearts," and which were at one time supposed 

 to be true hearts. " Each pseudo-heart is divided into a 

 narrow, elongated, external portion (the so-called ' ventricle ') 

 which communicates, as Dr Hancock has proved, by a small 

 apical aperture, with the pallial cavity ; and a broad, funnel- 

 shaped, inner division (the so-called ' auricle ') communicating, 

 on the one hand, by a constricted neck, with the so-called 

 ' ventricle ; ' and, on the other, by a wide, patent mouth, with 

 a chamber which occupies most of the cavity of the body 

 proper, and sends more or less branched diverticula 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 also to convey away the reproductive elements, the organs 

 for which are developed in various parts of its walls. By 

 Woodward, the pseudo-hearts are regarded as oviducts, and it 

 is stated that they have been found to contain mature ova, so 

 that there can be little doubt but that this view of their nature 

 is the correct one. 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 

 true vascular system and a distinct heart are present in some, 

 at any rate, of the Brachiopoda, but this subject is still involved 

 in considerable obscurity. In Terebratula the heart is in the 

 form of a unilocular, pyriform vesicle, placed on the dorsal 

 surface of the stomach. 



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 

 greater development, and consists of a gangliated oesophageal 

 collar. 



The sexes are said to be ordinarily distinct, but in some 

 cases they appear to be united in the same individual. The 

 development of the Brachiopoda is still shrouded in considerable 

 obscurity, but in some cases the young have been observed to 

 move from place to place, either by protruding their ciliated 

 VOL. i. s 



