. A manual of zoology. Zoology. II. ACEPHALA: PROTOCHONCHLE 323 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 wdth 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 tra
. A manual of zoology. Zoology. II. ACEPHALA: PROTOCHONCHLE 323 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 wdth 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. -. a. Ct b t p Fig. 326.—Anatomy of Nucula (after Drew), aa, anterior adductor; b, byssal gland; c, cerebral ganglion; ct, ctenidium;/, foot; h, heart; /, labial palpus; o, statocyst; p, pedal ganglion; /la, posterior adductor; s, stomach; t, appendage of palpus; v, visceral ganglion. Sub Order I. DIMYARIA. Two equally developed adductors. The taxodont have ctenidia, a broad foot, pleural and cerebral ganglia separate, and gonads emptying through the nephridia, all points which show them primitive. Nucula *Leda, but filibranch. Scapharca* Argina.'* Sub Order II. ANISOMYARIA. myaria) or wanting (Monomyaria). the families laclv a hinge. To the Yoldia* The Arcid^ are also taxodont, SOLEMYID^. Anterior adductor rudimentary (Hetero- With the exception of , all Heteromyaria belong the Mylilid.^e, or mussels, with strong byssus and shells pointed anteriorly. Modiola,'''' Pinna * Mytihis edidis;* eaten in Europe, but occasionally poisonous. Lithodomiis* bores into stone. The includes the pearl oysters of the East and West Indies (Meleagrina). OsTR^: and , monomyarian. The Ostrceidae, or oysters, usually attached by the right valve. The Pectinidce, or scallops, are free-swimming and have highly developed green eyes on the edge of the Please note that these images are extracted from scanne
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