S26 MOLLUSCOUS ANIMALS. 



the mode of aeration and of introducing the water-currents by which 

 both air and nutriment are supplied to the creature. The 4th class, 

 Conchifera, representing the type of vegetative or absorbent nutrition, 

 iias as its two sub-classes Lamellibranchiata and Palliobranchiata, in 

 the former of which the aeration is effected by distinct lamellae or 

 folds in which the blood-vessels are distributed, and the edges of the 

 mantle are more or less separated with usually a large foot which can 

 he protruded beyond the bivalve shell ; in the latter the aeration is 

 accomplished in the mantle itself, in which the vessels are distributed, 

 and instead' of the currents being produced chiefly by marginal fringes 

 or cilia around the incurrent orifice where the mantle is united to 

 ■form tubes, a singular development, apparently homologous with the 

 ■foot, of twoj generally spiral, ciliated arms serves to draw in the water 

 required both for nutriment and aeration causing it to pass over the 

 '!Iiantle and through the alimentary canal. There is unquestionably 

 a striking analogy between the arms of Palliobranchiata and the 

 ciliated border of Polyzoa, the cases in which the latter assumes the 

 horse-shoe form establishing their common nature, and it may justly 

 be inferred that this is the lowest form of the contrivance for intro- 

 ducing water containing both air and food, which consists of minute 

 animalcules and decaying animal and vegetable matters, into the sys- 

 tem of acephalous Mollusks, but that it does not indicate the closest 

 relationship between Palliobranchiata and Polyzoa may be concluded 

 from the seemingly superior importance of the points in which they 

 differ, and which connect thom respectively with two well-marked 

 classes. 



The fifth and last class, Tunicata represents the lowest development 

 of the Molluscan type, and is characterized by the sacciform mantle 

 and the aeration being effected either simply by the water passing 

 over the interior of the sac in a perpetually renewed flow, or in the 

 higher forms by means of a fenestrated special organ over which the 

 vascular system is distributed, within and around which the water is 

 made to pass. In the lower sub-class, Polyzoa, the external surface 

 is firm and translucent and the incurrent opening is enlarged, (as if 

 by a protrusion of the branchial sac of the Ascidioid Tunicata), with 

 a border of m.inute rays which are finely ciliated, and it is even pro- 

 bable that here the aerating process is chiefly carried on. In the 

 higher sub-class, Tunicata proper, the outer covering is very generally 

 flexible, sometimes soft and transparent, the branchial sac is included, 



