Introduction to Animal Morphology. 



335 



in Limulus it has seven pair of ostia, and sends not only an 

 arterial stem fore and aft, but also four branches from four of 

 its intermediate chambers : these divide in the abundant in- 

 terstitial tissue, and end in lacunae. In Isopoda there are 

 fewer ostia (three in Tanais), and the heart stretches forward 

 to near the head. It extends towards the tail in Amphipoda, 

 where the ostia are also few (three, Phronima, seven, Gamma- 

 rus). These have one anterior aorta, or an anterior and a ru- 

 dimental posterior. In the larvae of the Podophthalms tin-re 

 are two venous ostia ; but as development proceeds, the num- 

 ber of chambers (five) and ostia may enlarge, as in Stomapoda, 

 and the heart then gives off anterior, posterior, and lateral 



Circulatory system of Lobster ; /, pericardium ; /;, heart ; a, middle anterior 

 aorta ; a", hepatic artery; a", ventral artery; a, posterior aorta ; ?>, ventral venous 

 sinus ; v, veins from head ; v", veins from the limbs ; b, gills ; v.b, branchial veins. 



branches, or as in Schizopoda and Decapoda the organ be- 

 comes concentrated, reticulated within, not divided into 

 chambers, and with its five pair of ostia not in a I nit in 



a group. The anterior aorta is short, and ^i\ t s off three 

 branches, a middle to the brain and eyes, ami lateral to the 

 digestive and reproductive organs. The posterior aorta 

 > into dorsal and ventral branches ; the former to the 

 muscles of the trunk ; t; which divides into anterior 



and posterior branches, sending a vessel to each limb. These 

 branches end in capillaries, communicating, , with 



the periviseeral lacunx, from whence the. blood en 



