Ixxxvi Introduction. 



proctuclious, tlie anal outlet is very usually in close relation to the 

 respiratory inlet. The hepatic organ is very various in shape, but 

 is usually, as in most water-breathing- animals, largely developed. A 

 heart is usually but not always present ; it is, when present, always 

 systemic, receiving blood, when afferent veins are present, from the 

 aerating and renal organs, or from the general lacunar system when 

 no such vessels exist, and propelling it by an aorta to the main 

 organs of animal and vegetable life respectively. In many aquatic 

 Mollusca, the external water can find its way by variously situated 

 apertures, so as to become directly intermingled with the blood ; in 

 others, a multi-ramified water-vascular system appears to spread 

 itself throughout the body, without becoming directly continuous 

 with the blood-vessels. In some cases no specialized respiratory 

 organs are present ; in a few Mollusca aerial respiration is attained 

 to, in most it is aquatic. The renal organ may be represented by a 

 simple non-glandular sac, which communicates internally with the 

 lacunar blood-vascular system, and externally with the circum- 

 ambient medium. When its walls are clothed with glandular cells, 

 it receives an abundant supply of venous blood, some of which is 

 passed onwards to the aerating organs, and some sent directly to 

 the heart. 



The nervous system may be reduced to a single ganglion, as in 

 Polyzoa and Tunicata, and in the former class organs of special 

 sense are wanting, except occasionally as rudiments. 



Reproduction may be either sexual or asexual ; and in the Po- 

 lyzoa polymorphic zooids are produced by gemmation. The ova 

 undergo entire segmentation, except in the Cephalopoda ; and with 

 the exception of that class, and a few Gasteropoda, of the pulmonate 

 order mostly, the embryos go through metamorphosis subsequently 

 to being set free from the egg. 



The great majority of Mollusca are water-breathers, and marine 

 in habitat ; some however are fluviatile, or lacustrine ; and a few 

 are terrestrial and pulmonate. A few Mollusca are parasitic. 

 Entoconcha mirah'ilis inhabits the perivisceral cavity of a Synapta; 

 in the genus Eidima, which preys upon Holothurioidea, some species 

 are ento-, others ecto-parasitic. StT/lifer lives ecto- or pseudo- 

 parasitically attached to the soft tissues clothing the exterior of 

 Asteriae and Eehinoidea and Holothurioidea, as also within their 

 digestive tract. Crenella infests Tunicata; and Vulsella, Gastro- 



