l62 ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 



glandular walls ; its communication with the quadrangular dilatation 

 of the intestine is so wide that it may almost be considered as a 

 diverticulum thereof, and it extends back as far into the spire of the 

 shell as any of the viscera. 



The rectum is of a pinkish colour, and is richly ciliated internally. 



Circulatory System (figs. 2, 3, 4). — The heart resembles that of 

 Firolo'ides, and consists of an auricle iji) and a ventricle {v). It lies 

 parallel to the rectum, with the auricle forwards at the base of the 

 mantle-cavity, and the animal is therefore prosobranchiate. The 

 aorta proceeds from the apex of the ventricle, and immediately after 

 its origin divides into two branches, one of which runs backwards to 

 the visceral mass, while the other passes forwards close beneath the 

 stomach, until it terminates in the buccal mass. After passing over 

 the subcesophageal ganglia, it gives off a downward branch to the fin, 

 but I did not observe the peculiar termination of this artery which 

 obtains in Firolo'ides ; this perhaps may be accounted for by the 

 greater muscularity of the fin in Atlanta, rendering it less transparent. 

 The venous blood has no distinct channel, but returns to the heart by 

 the cavity of the body. The returning current of blood-corpuscles is 

 very obvious ; they seem to pass quite freely in all directions round 

 the intestine, aorta, and nervous centres, the general tendency being 

 always backwards towards the heart. 



Respiratory System. — Very distinct gills are figured by Eydoux 

 and Souleyet in most Atlanta;, but their presence in this species was 

 decidedly exceptional, the majority of specimens presenting no trace 

 of them. Once I noticed a bundle of long branchial filaments de- 

 pending from the wall of the mantle-chamber ; and in another case 

 rudimentary and undeveloped short processes of the same kind were 

 to be seen ; they contained canals, through which a small portion of 

 the returning venous blood was diverted, fig. 4. 



The Mantle is very well developed ; a peculiar thickened and ciliated 

 band crosses it transversely, and seems to be the homologue of the 

 subspiral ciliated band in Firolo'ides {d). 



Contractile Sac. — This resembles the corresponding organ in Firo- 

 loides ; it lies between the rectum and the heart, and opens into the 

 bottom of the chamber of the mantle by a well-defined oval aperture, 

 figs. 3 and 4 c. 



Generative Organs. — The ovary or testis is an elongated mass cor- 

 responding to the liver, and occupying the inner and right half of the 

 visceral mass, fig. 3 t. 



The Ovary consists entirely of a mass of ova in course of develop- 

 ment, with their characteristic germinal vesicle and spot. The oviduct 



