GASTROPODA PROSOBRANCHIATA. 43 



By far the majority of Gastropoda are marine ; the Basommato- 

 pliora and some Prosobranchiata (Paludina, Valvata, M'-lnnm, 

 X i' retina, etc.) inhabit fresh-water. Many Littorina, CV/-/Y///". 

 Mi'lania, etc., live in brackish water. The Cyclostomidce, and the 

 Stylommotophora among the Pulmonates, are terrestrial. Further, 

 many branchiate Gastropods are able to live for some time out of 

 water in dry places; in such circumstances they are withdrawn into 

 their shells, the opening of which is closed by the operculum. 

 Almost all move by creeping ; some, however, as Strombus, jump ; 

 others, as Oliva and Ancillaria, swim excellently by the aid of the 

 lobes of their foot. Some marine forms, as Magilus-, Vermetus, etc., 

 are fixed by their shells; a few only are parasitic, as Ktylifer on 

 sea-urchins and starfishes, Entoconcha mirabilis in Synapta. 



The method of nutrition differs as much as the habitat. Many, 

 especially the Siphonostomata, are voracious predatory animals, and 

 prey on living animals; some branchiate Gastropods, as Hurex and 

 ^Yuftca, with this object bore into the shells of Molluscs; several 

 (tifrombus, Buctinwm) prefer dead animals. An equally large 

 number, viz., almost all Pulmonates and holostoniatous branchiate 

 Gastropods, feed on plants. 



Order 1. PROSOBRANCHIATA.* 



Dioecious branchiate Gastropods with shell, and with gills in front of 

 the heart. 



Behind the usually distinctly separated head lies the respiratory 

 (mantle) cavity, into which the rectum, kidney, and oviduct open. 

 In rare cases two gills are present, as a rule the right gill is absent. 

 The branchial veins enter the heart from the front. Cerebral, pedal,, 

 pleural and visceral ganglia are present. The males are, as a rule, 

 more slender, and are easily recognized by the large penis placed 

 on the right side of the anterior part of the body. In the generative 

 organs, the accessory glands are usually absent. The eggs are sur- 

 rounded by albumen and laid in capsules, which are frequently fixed 

 to foreign objects ; more rarely they are attached to the foot and 

 carried about (Janthina). 



* Fr. Leydig, " Ueber Paludina vivipara." Zdt. fur.wiss. Zool., Tom II., 

 1850. 



E. Clapar&de, "Anatomic und Entvvickelungsgeschichte dcr Neritina flu- 

 viatilis." Miillrr's Archil-., 1S.~>7 '. 



H. Lacaze-Duthiers, " M&noire sur le systeme nerv.de 1'Haliotideet Memoire 

 sur la Poupre.'' Ann. (lets tie. A<tt., Turn XII. and XIII 



N. Bobretzky, I.e. 



