BRACHIOPODA. 83 



running on the oesophagus, and gives off several lateral arterial 

 trunks. The vascular system is not closed, but is in connection with 

 a blood sinus surrounding the alimentary canal, with the lacuna 

 of the viscera and with a well-developed system of lacunae in the 

 mantle and arms. In the latter the blood is brought into close 

 osmotic relation with the water, over a large surface; the inner 

 surface of the mantle and the spiral arms are, therefore, correctly 

 regarded as respiratory organs. 



Excretory organs. Two, rarely four, canals, which are provided 

 with glandular walls and open on each side of the intestine with a 

 funnel-shaped aperture (fig. 554 T r) into the body cavity, and on 

 either side of the mouth to the exterior, are to be regarded as kidneys 

 (corresponding to the segmental organs of Annelida.) They function 

 at the same time as generative ducts, 

 and were called oviducts by Hancock. 



The nervous system consists of a 

 circurncesophageal ring on which two 

 small supraoasophageal ganglia are in- 

 serted. The subcesophageal ganglionic 

 swelling of the ring is, however, much 

 larger, and from it nerves pass out to 

 the dorsal mantle lobe, the arms and ad- 

 ductor muscle^, and to two small ganglia 

 which supply the ventral mantle lobe 



FIG. 557 a. Larva of Lingula (after 



and the peduncular muscle with nerves. Brooks). T, Tentacles : o, mouth/ 



Sense Organs are not known with Cer- *>> alimentary canal: Af, anus; L, 



liver ; St, rudiment of peduncle. 



tainty. 



Generative organs. In all probability most Brachiopoda, as 

 Discina, Thecidium and Terebratulina are dioacious. The sexual organs 

 consist of thick yellow bands and ridges which have a paired arrange- 

 ment and project from the body cavity into the lacunae of the mantle, 

 and are there considerably ramified. The eggs pass from the glands 

 into the body cavity, and are conducted to the exterior by the ovi- 

 ducts (excretory organs) whose funnel-shaped internal openings have 

 already been mentioned. 



Development (fig. 556). After a total segmentation a kind of 

 gastrula is formed, usually by invagination, and the archenteron 

 (Argiope) becomes divided as in Sagitta into a median cavity, and 

 two lateral diverticula which are constricted off and give rise to the 

 body cavity (fig. 556 a, b). The oval larva then elongates and 

 becomes divided by constrictions into three segments (fig. 556 b, c), 



