68 LAMELLIBRANCH MOLLUSKS. 



LESSON V. 



Class of Mollusca Acephala, or, Lamelliuranchiata. — 



Organization — Classification. 

 Family of Ostkacea. — Classification — Oysters — Gryphcea—- 



Pecten — Lima — Malleus — Anomia — Spondylus — Perna — 



Etheria — Avicula — Pearls — Pearl fishery — Pinna — Arca-^ 



Pectuncvlus — Trigonia. 

 Family of Mytilacea. — Mussels (My i\\us)— Modiolus — A7io- 



donta — Unio. 

 Family of Chamacea. — Tridacna — Chama — Isocardia. 



CLiVSS OF 3MOIiIiUSCi\. IiAJyE£Z.X.IBRAKrCHIikTi!L. 



1. All the mollusks we have heretofore considered have a dis- 

 tinct head ; those we have yet to mention are without this part, 

 and their whole organization is of the simplest kind. The 

 mouth is always concealed at the bottom of the mantle, or be- 

 tween its folds : they have neither teeth nor eyes. The nervous 

 system is very simple, and the organs of locomotion are very 

 incomplete or lamellibranch. 



2. In animals which form the class of acephalous mollusks 

 (called by Lamarck Mollusca conchifera), the mantle is very 

 large and folded in two, so as to enclose the body, as the leaves 

 of a book are enclosed by its covers. Sometimes these two leaves 

 are free at their inferior edge, sometimes united so as to con- 

 stitute a tube (fig- 88, ts, ti). A shell, composed of two 

 pieces, called valves, covers this mantle entirely or in part, and 

 at its superior part there is a hinge furnished with an elastic 

 ligament, the play of which causes the valves to gape whenever 

 the muscles {ma^ mp), which extend from one valve to the other, 

 do not contract so as to keep them shut. The branchioe (6r) 

 are in form of great leaves, transversely and regularly striated ; 

 they are always four in number, and placed between the in- 

 ternal face of the mantle (or pallial membrane) and the body of 

 the animal. The mouth is at one of the extremities of the body, 

 and has on each side of it two small triangular bodies (/), 

 which are formed by the extremities of the lips and serve as 

 tentacula ; the stomach, liver, and other viscera, are lodged 

 between the mouth and anus, and beneath the heart (co), 

 which is situate on the back ; and the lower part of the body 



1. What is the disting-uishing feature of the lamellibra'nch mollusks? 

 (Lamellibranch; from the Latin, lamella, a thin plate, and hranchics, gills.) 



2. What are the general characters of acephalous mollusks ? (pronounced 

 a-kef-alus ; from the Greek, a, without, and kephale, head. 



