. A textbook of invertebrate morphology [microform]. Invertebrates; Morphology (Animals); Invertébrés; Morphologie (Animaux). TYPE MOLLUSC A. 303 duct {vd), having apparently no relation to the nephridia and opening into the mantle-cavity to the right side of the anus. In the more primitive Gasteropoda, however, such as Haliotis, Fissurdla, and Patella, the nephridia, as in the Solenogastres, serve as reproductive ducts; and it has been suggested that the special reproductive duct of the remaining Gasteropods may represent the left uephridiura, which is usually described as having disappeared.
. A textbook of invertebrate morphology [microform]. Invertebrates; Morphology (Animals); Invertébrés; Morphologie (Animaux). TYPE MOLLUSC A. 303 duct {vd), having apparently no relation to the nephridia and opening into the mantle-cavity to the right side of the anus. In the more primitive Gasteropoda, however, such as Haliotis, Fissurdla, and Patella, the nephridia, as in the Solenogastres, serve as reproductive ducts; and it has been suggested that the special reproductive duct of the remaining Gasteropods may represent the left uephridiura, which is usually described as having disappeared. The reproductive duct, especially in hermaphrodite forms, has developed in connection with it accessory glandular structures as well as external copulatory organs, the whole reproductive system becoming highly com- plicated. An account of the more important arrangements will be more satisfactorily given in connection with the various orders. 1. Order Frosobranchia. The Frosobranchia are, with very few exceptions, marine Gasteropods, provided with well-developed shells, which are usually spirally coiled, the height of the spiral varying, however, in different forms. In some, such as Patella and Fissurella, the shell has a simple conical form, without any indication of a spiral; and since these forms in many respects show primitive charac- ters, it might be supposed that this type of shell was also primitive. These very forms, however, show also that asymmetry of parts, which is character-Pig. 135.—Shells of Puoso- istic for the Gasteropods, and which buanch Gasteuopods. accompanies the rotation of the mantle- cavity, and furthermore, in Fhsiirella at least, a distinct indication of a spiral coiling, is present in the shells of young animals. It seems more probable, accordingly, that these conical shells are to be regarded as secondary modifi- cations of an originally spirally-coiled shell. The mantle-cavity is situated in front of the well-devel- oped visceral hump, and is
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