142 THE GASTROPODA 



the families Eulim.Iae (including Stylifer, parasitic on Asterids, 

 Echinids, and Crinoids) and Entoconchidae, including Entosiphwi, 

 Entocolax, Entoconc/ia, and Enteroxenos, all parasitic in Holothurids. 



Some thirty thousand species of Gastropoda have been enu- 

 merated, of which twenty thousand belong to the present epoch and 

 are distributed in every region of the globe. Of existing species 

 more than twelve thousand are branchiate forms. Some marine 

 species are found at a depth of over 2500 fathoms, and some 

 Pulmonata live in the Himalayas at a height of nearly 17,000 feet 

 above the level of the sea. Some freshwater Gastropoda (Hydro- 

 biidae, Basommatophora) exist at a depth of 180 fathoms below the 

 surface of certain lakes, e.g. Lake Baikal; others live in subterranean 

 waters, and some Pulmonates are found in caverns into which the 

 daylight does not penetrate. Palaeontology shows that these 

 animals were already in existence in the Cambrian period, at the 

 commencement of the Palaeozoic epoch. 



The size of Gastropods varies from a fraction of a millimetre to 

 more than fifty centimetres. The largest forms are found not only 

 among the testaceous species, such as Fusus, Tritonium, Ancistromesus, 

 Strombus, etc., but also among the naked forms : Tethys, for example, 

 is more than thirty centimetres in length, and some species of Den- 

 dronotus as much as twenty-five centimetres. 



VI. SYSTEMATIC REVIEW OF THE SUB-CLASSES, ORDERS, AND 

 FAMILIES OF GASTROPODA. 



The class Gastropoda includes two well-defined sub -classes, 

 Streptoneura and Euthyneura. 



SUB-CLASS I. STREPTONEURA, Spengel 

 ( = Prosobranchia, Milne-Edwards = Cochlides, von Jhering). 



These are dioecious Gastropoda, with the exception of a few 

 aberrant genera, and are characterised by the maximum torsion 

 exhibited by the visceral mass and visceral commissure, the latter 

 being always twisted into a figure of eight (Fig. 124, VII, IX). 

 The right moiety of this commissure is situated above the digestive 

 tube, and is known as the supra-intestinal ; the left moiety is situated 

 below the digestive tube, and is known as the infra-intestinal. The 

 pleural ganglia are often united to the opposite branch of the 

 visceral nerve by an anastomosis of the pallial nerve, this condition 

 constituting " dialyneury " (Fig. 123, A, di', di") : or there may be a 

 direct connection by means of a longer or shorter connective pass- 

 ing from the pleural ganglion to the ganglion borne on the visceral 

 branch of the opposite side ; this constitutes " zygoneury " (Fig. 

 123, B, C, zy, zy"). Zygoneury is more frequently found on the 



