THE ANATOMY OK MKCALATHACTUS — KKSTEVEN. 441 



The aorta-cejihnUca is very much tht; hirger of th(! two urterics, 

 jiiul its name implies its fuiictiun ; the main branch Hes along the 

 left side of the body cavity. 



The aorta-visceralis is a very small vessel for so large a mollusc ; 

 it passes up the axis of the visceral coil, giving off small branches 

 in its course. 



The vena-maxima is a large venous sinus, which places the 

 lacun;e of the visceral coil in communication with the rec^a^ sinu>i, 

 and gives off numerous branches to the glandular mass of the 

 nephridium. 



The vena-cephalica is a very much reduced posterior prolongation 

 of the bod}' cavity, and places that large sinus in communication 

 with the pericardium and nephridium. The nephridial branch 

 gives off numerous branches which lie on the surface of the 

 glandular mass and between the branches of the vena-maxima. 



The vena-cava is a short sinus placing the pericardial gland in 

 communication with the auricle. 



The efferent branchial vein forms a distinct ridge along the left 

 side of the ctenidium ; its cavity is well defined. 



The venai-'pericarcliales are two sinuses connected with the 

 posterior end of the pericardium, and placing that cavity in 

 communication with the lacunas of the visceral coil. 



For the course of circulation, see p. 429. 



6, The Nervous System. 



As absolutely nothing is known of this system in M. aruanus, 

 a generic summary of the characters is impossible ; it is, there- 

 fore, unnecessai'y to repeat the description given, under M. 

 maxiimcs. 



7. Reproductive Systems. 



Male. — The gonad is not large ; it has the position usual in 

 the Monotocardian Prosobranchs. 



The vas-deferens is coiled for a short distance, but for the 

 greater part of its length takes a direct course. In the posterior 

 half of the pallial chamber it lies on the mantle, thence to the 

 penis it lies on the body. 



The penis is large, oval in transverse section, attached on the 

 right side of the body, just behind the tentacle lobe. It is 

 carried folded back and appressed to the body on the right side, 

 in the pallial chamber. 



Female. — The gonad is macroscopically similar to that of the 

 male. 



The uterus is large, situated in the pallial cavity on the right 

 side between the rectum and body, and it extends for a short dis- 



