26 STUDIES IN BIOLOGY. 



mantle in consequence in M. edulis (Fig. 31) than in M. latus (Fig. 

 29) . It is noteworthy, in connection with the presence of the marginal 

 artery, that the border of pallial mnscles is mnch wider in M. latus 

 than in M. edulis (Figs. 29 and 31) . Instead of a marginal artery, 

 however, there is in M. edulis a marginal sinns or vein following the 

 edge of the mantle (m s, Fig. 32) : this marginal sinus is not present 

 in M. latus. 



As to the disposition of the pallial arteries, there is great dif- 

 ference between the three species M. latus, edulis, and magellanicus . 

 In M. edulis and -magellanicus the whole of the mantle is supplied by 

 pallial arteries springing from the anterior side of the aortic bulb in 

 the former, and from the anterior aorta in the latter (Figs. 22 and 

 23) ; the arteries from the anterior side of the aortic bulb in M. 

 latus supply only the anterior half of the mantle (Fig. 21) . This 

 difference is probably due to the disposition of the muscles. In M. 

 edulis and magellanicus the retractors are so arranged as to form an 

 almost continuous wall below the pericardial region, thus forcing the 

 pallial arteries to pass round the anterior end of this barrier. In 

 M. latus the lower side of the pericardial region is quite free from 

 any such obstruction, on account of the separation of the middle and 

 posterior retractors. This, together with the great development of 

 the posterior aorta, renders it natural that in M. latus the posterior 

 halves of the mantle-lobes should be supplied from behind the aortic 

 bulb or from the posterior aorta, as is the case. With regard to the 

 continuation of the anterior aorta and also of the posterior aorta into 

 a circumpalliai circuit, this would seem to be unique, so far as I have 

 been able to see. No account of any such arrangement has been de- 

 scribed in any Lamellibranch. On the contrary, Gegenbaur (9) 

 gives a very different arrangement as the normal condition in 

 Lamellibranchs. He says that " the anterior arterial trunk [anterior 

 aorta] passes as far as the region of the mouth, where it gives off 

 branches and opens into wide haemal spaces. The posterior arterial 

 trunk [posterior aorta or pericardial artery], the length of which is 

 dependent on the development of the hinder portions of the mantle, 

 which represent the siphons, also jjasses into haemal spaces or lacuna" 

 [Italics mine.] 



Venous System. 



This, in M. latus, is very difficult to work out, as it consists in 

 great part of lacunae or haemal spaces, and the softness of the tissues 

 demands the greatest care in the operation of injection, else the in- 

 jection may take almost any course. Milne-Edwards (10), in giving 



