304 Ilonograph of the Gasteropoda. 



which an intestine extends, winding through the great liver and ovary, 

 passing through the bronchial cavity along the base of the gills, term- 

 inating at the branchial opening, usually on the right side of the head. 

 In some genera the stomach is armed with calcareous spines, or, as in 

 Bulla, with three large grinding plates, concave outwardly. The 

 liver is extremely large and lobulated ; the ovaries are large, granu- 

 lated masses, enveloping the intestine opening into a large oviduct 

 (which is also the uterus of the ovo viperous species), running parallel 

 to the inner edge of the rectum, separated from it by a slender renal 

 duct, and all opening at the same point ; the testis forms a more com- 

 pact gland farther back (in the apex of univalve shells), sending its 

 vas deferens close along the oviduct. The heart is systematic, and has 

 one auricle (or in Haliotis, chiton and Fissurella two) receiving the 

 oxygenated blood from the gills, which, flowing into the ventricle, is 

 thence distributed to the body through an aorta extending from its 

 apex. The eyes never exceed two in number, but are perfectly formed, 

 having a transparent cornea, a large, sj^herical, crystalline lens, with a 

 small intervening space for the aqueous humor, and a circular pupil- 

 lary opening left by the choroid or pigmental layer. The organ of 

 hearing consists of two round vesicles, adhering to the anterior oesoph- 

 agal ganglia, and containing numerous calcareous, oscillating otolites. 

 The shell varies greatly in form and position, according to the group, 

 but is formed in all from the calcification of shells formed in layers 

 under the epidermis, by the edge of the mantel, and irregularities in 

 form of its edge, produce corresponding spines, tubercles, etc., on the 

 shell, which the animal is able to absorb again when they become in- 

 ternal by the spiral growth of the whorls. There are usually three 

 distinct layers in the univalve shells. All the orders are hermaphro- 

 dite, except the Pedmibranchiata, in which the sexes are distinct. The 

 7iervous system is greatly more centralized than in LameUibranchiata, and 

 the principal ganglia approximated to the head, there being usually a 

 great pair of supra oesophagal ganglia, or brains, supply the eyes, ten- 

 tacles, etc., and a great suboesoj)hagal mass of ganglia giving off nerves 

 to supply the foot, the gills, the viscera, etc., connected by cords form- 

 ing a nervous collar." 



The Gasteropoda are divided into two sub classes, the Heteropoda or 

 Niicleobranchiata, which includes only a single order, and the Gastero- 

 poda proper. 



The Gasteropoda form two natural groups ; one breathing air {pul- 

 r>ibniferd) , the other water (hramchifera) . None of the pidmonifera 

 belong to the Cincinnati Group. The branchifera are always provided 

 with a shell whilst in the egg, sufficient to conceal them entirely when 



