312 COMPARATIVE ANATOMY. 



general arrangement is that the large trunks run dorsally along the 

 euteron ; in this they may be seen to resemble the arrangements 

 which are found in the Vermes. But the special points in this 

 system of organs require further investigation. 



A saccular organ lying above the stomach is regarded as the heart ; 

 this receives a vascular trunk which runs from in front above the 

 oesophagus, and gives off lateral branches. The former is regarded 

 as an afferent vessel (vein). It seems to collect the blood from 

 lacuna?, which are placed around the enteric canal. Two lateral 

 vessels given off from the heart are united in the Testicardines 

 (Waldheimia) for a short distance. In the Ecardines (Lingula) they 

 are not given off till later from a median longitudinal trunk which 

 passes backwards on the enteron. Two arterial trunks, which have 

 been called aortas, soon divide into two branches, one of which passes 

 forwards and the other backwards. The anterior one represents 

 the dorsal pallial artery, which divides into a median and a lateral 

 branch, and supplies the mantle and the organs which lie in it. 

 Smaller arteries are given off from the lateral branch to the 

 lacunas of the mantle; they pass to the edge of it, and then 

 open after having divided several times. The hinder branch of 

 the aorta also divides into two arteries. One runs along the middle 

 line, and forms, with its fellow of the other side, an arterial trunk 

 which passes to the stalk. The other artery turns forwards, and 

 again divides into two branches in the ventral lobes of the mantle 

 where it ramifies, in just the same way as the dorsal pallial artery. 

 On the two pairs of pallial arteries there is a pouch-like appendage, 

 or accessory heart. The blood seems to pass from the ends of the 

 arteries into wider lacunas which are placed in the mantle, as well as 

 between the viscera, and around the muscles ; and these are con- 

 nected with a complicated system of canals, which traverses the 

 arms, and is divided into an efferent and an afferent portion. 



As the mantle is a secondary structure, its blood-vessels may be 

 regarded as being so also. The pallial arteries are, therefore, of little 

 importance, and the large trunks which accompany the enteron 

 become of greater morphological significance. The heart appears to 

 be a unilateral enlargement of the longitudinal trunk, as are also 

 the accessory hearts of the pallial arteries. 



Excretory Organs. 



§ 245. 



The excretory organs found in the Vermes, and adapted to the 

 presence of a coelom, are also found in the Brachiopoda, where they 

 have essentially the same characters. Like the looped canals of the 

 Annelides, these organs have an internal and an external orifice, so 

 that I have no hesitation in regarding these structures as homologous, 



