ANATOMY OF THE SNAIL. 



261 



is, in those species inhabiting a spiral shell, asymmetrical 

 and wound in a spiral, the visceral mass extending into the 

 apex of the shell. In the Nudibranchs (Figs. 195, 197), and 

 the slug, the body being naked is symmetrical 

 on each side. 



The digestive tract is doubled on itself, the 

 vent ending on one side of the mouth. In 

 some Nudibranchs the intestine has numerous 

 lateral offshoots, or gastro-hepatic branches, 

 which resemble similar structures in the Plana- 

 rian and Trematode worms. A heart is always 

 present, except in the parasitic Entoconcha, 

 and sometimes, as in Chiton, Neritina, and 

 Haliotis, it is perforated by the intestine. In 

 some genera there are two auricles to the heart, 

 but as a rule but one is present. The Gastro- 

 pods breathe by gills either free, or contained 

 in a cavity in the mantle, while in the land- 

 snails (Pulmonata) the air is breathed directly by a lung-like 

 gill in a mantle-cavity. The kidney is single. The sexes 

 are either distinct or united in the same individual. 



An excellent idea of the structure of a typical Gastropod 

 may be obtained by a dissection of Natica (Lunatia) heros. 

 This is a large mollusk, common between tide-marks from 

 Labrador to Georgia. On taking it up the student will 

 notice the large, round, swollen, porous foot, from which 

 the water pours as if from the "rose" of a watering-pot. 

 The shell is large, composed of several whorls, with a small 

 flattened spire or apex. The aperture is large, lunate in 

 shape, and can be closed by a large horny door or oper- 

 culum. (In some mollusks, Natica, Turbo, etc., the oper- 

 culum is of solid limestone, and small ones are used as " eye- 

 stones," being inserted in the eye and moved about by the 

 action of the lids, thus cleansing the eye of irritant particles 

 of dust, etc.) 



The animal should then be placed in a dish of salt water, 

 and its movements observed. There are but two short, 

 broad, flattened tentacles, situated on a flap or head-lobe 

 (prosoma) of the mantle or body-walls. No eyes are present 



