II. ACEPHALA: PROTOCHONCHLE 

 Order I. Protochonchiae. 



The primitive character of these forms is shown by the structure of the 

 gills, which are either ctenidia (Protobranchiata) or filamentary (Fili- 

 branchiata) , yet here and there, as in the scallops and oysters (Pseudo- 

 lamellibranchiata), the fusion of gill filaments is already begun. Hinge 

 and ligament are symmetrical with regard to the umbo, or vary little 

 from symmetry. Hinge teeth may be lacking, and the ligament is 

 wholly or in part internal. The mantle edges are free, and rarely is there 

 the first trace of fusion to form a siphon. 



FIG. 326. Anatomy of Nueula (after Drew), aa, anterior adductor; b, byssal 

 gland; c, cerebral ganglion; ct, ctenidium;/, foot; h, heart; /, labial palpus; o, statocyst; 

 p, pedal ganglion; pa, posterior adductor; s, stomach; t, appendage of palpus; v, visceral 

 ganglion. 



Sub Order I. DIMYARIA. Two equally developed adductors. The 

 taxodont NUCULID^E have ctenidia, a broad foot, pleural and cerebral ganglia 

 separate, and gonads emptying through the nephridia, all points which show them 

 extremely primitive. Nueula,* Leda,* Yoldia* The ARCID^E are also taxodont, 

 but filibranch. Scapliarca,* Argina.* SOLEMYID^E. 



Sub Order II. ANISOMYARIA. Anterior adductor rudimentary (Hetero- 

 myaria) or wanting (Monomyaria). With the exception of SPONDYLID^E, all 

 the families lack a hinge. To the Heteromyaria belong the MYI.ILID.S, or 

 mussels, with strong byssus and shells pointed anteriorly. MoJiula,* Pinna,* 

 Mytilns edulis;* eaten in Europe, but occasionally poisonous. Lifhodomus* 

 bores into stone. The AVICULID^E includes the pearl oysters of the East and 

 West Indies (Meleagrina) . OSTR^ID^ and PECTINID.*:, monomyarian. The 

 Ostrasidas, or oysters, usually attached by the right valve. The Pectinidas, or 

 scallops, are free-swimming and have highly developed green eyes on the edge of 

 the mantle. 



