142 THE GASTROPODA 
the families Eulimidae (including Stylifer, parasitic on Asterids, 
Echinids, and Crinoids) and Entoconchidae, including Lutosiphon, 
Entocolax, Entoconcha, 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- 
biudae, 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 Musus, Tritoniwm, Ancistromesus, 
Strombus, ete., but also among the naked forms: Tethys, for example, 
is more than thirty centimetres in length, and some species of Den- 
dionotus 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. 
Sus-Ciass 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 bea 
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. 
125, B, C, 2, zy). Zygoneury is more frequently found on the 
