i6 A SYNOPSIS OF ANIM/fL CLASSIFICATION. 



TYPE IV. MOLLUSCA. 



Soft-bodied, unsegmented animals, consisting typically of three 

 parts: head, foot, and visceral sac; this latter part is covered dorsally 

 by a duplicature of integument called the mantle, the outer layer of 

 which usually secretes a calcareous shell, consisting of one, two, or 

 several separate pieces, which sometimes depart from the original 

 bilateral symmetry; a complicated vascular system with a pulsating 

 heart and typically one or more pairs of plumose gills, the ctenidia, 

 which may be variously modified or entirely replaced by other organs; 

 nervous system composed of several pairs of ganglia; body cavity 

 restricted to a small open space surrounding the heart. 

 Class I. AMPHINEURA A small group of bilaterally sym- 

 metrical molluscs showing similar- 

 ity to the Turbellaria. They have 

 a nerve-ring around the mouth and 

 four parallel longitudinal nerve- 

 cords connected by transverse com- 

 missures. 



Order i. Solenogastres. Mantle and shell wanting. Body 



worm-like. x Proneomenia. 



Order 2. Chitones With a dorsal shell composed of 



eight transverse pieces. 



X Chiton. 

 Class II. PELECYPODA (Lamelli- 



branchiata) Bilaterally symmetrical, or, in a 



few cases, secondarily distorted, 

 owing to an habitual lateral posi- 

 tion; two lateral shells, usually 

 united dorsally. The single pair of 

 ctenidia seldom appears in a sim- 

 ple form, in most cases the separate 

 plates are elongated and filiform, 

 and are often doubled upon 

 themselves and united together in 

 such a way as to form two thin 

 lamellae upon each side. They 



