358 MOLLUSCA. 



The integument consists of a superficial layer of cylindrical cells 

 which are frequently ciliated; and of a connective-tissue dermis 

 which is inseparably connected with the dermal muscles. Calcareous 

 and pigment glands are placed in the integument ; they are specially 

 numerous at the edge of the mantle-fold, where they contribute to 

 the growth and peculiar colouring of the shell. 



The mantle (pallium) or integument of the visceral sac is thrown 

 into a continuous fold, which completely encircles the body at the 

 junction of the visceral sac with ihe head and foot. This fold is 

 the mantle-fold; it encloses between itself and the body a space 

 called the mantle- or pallial-groove. The mantle-groove is quite 

 shallow, and groove-like over the greater part of its extent, and in 

 the Placophora (Fig. 305 B) over its whole extent ; but in all other 

 forms it is especially deepened at one point. This specially deep 

 part of the mantle-groove is the mantle-cavity proper. The mantle- 

 cavity contains the ctenidium (in the Zygobranchiata the two ctenidia) 

 when there is a ctenidium, and generally the anus, renal orifice, and 

 generative opening. 



The free edge of the mantle-fold is thickened and may be provided with short 

 tentacles, pigment spots, and glands. In the Zygobrancliiata y and in one or 

 two other forms, there is a slit along the roof of the mantle-cavity in the 

 mantle-fold. This slit, the edges of which may be fused at one or more points, 

 allows of the exit of the spent water from the mantle-cavity. 



The siphon (Fig. 280, 22) is a kind of spout-like continuation of the mantle- 

 edge on the left side of the mantle-cavity. It occurs in many, principally 

 carnivorous, Streptoneura, and it conducts water into the mantle- cavity. 



The hypobranchial gland (Fig. 280, 17) is a highly glandular and generally 

 folded portion of the mantle-lining between the two gills and the rectum (Zygo- 

 branchs), or between the single gill and the rectum (most other Gastropods). 



In the typical Gastropoda, such as the whelk (Buccinum) or the 

 garden-snail (Helix), the visceral sac is covered by a univalve shell. 

 This shell has, to a certain extent, the same shape as the visceral sac 

 (Fig. 312), and can usually completely receive and protect the head 

 and foot when the animal is retracted. As a rule it is hard and 

 calcareous, consisting of the three layers, an outer periostracum, 

 a middle prismatic or porcellanous, and an internal nacreous layer : 

 the nacreous layer is not, however, always present in Gastropods, and 

 the porcellanous is of complex structure, consisting often of three 

 layers of laminae, which are themselves composed of prisms. Some- 

 times it is a delicate structure, horny and flexible, or it may have a 

 gelatinous or cartilaginous consistence, as in the Cymbuliidae, where, 

 however, it is not a mantle-shell, but a subepidermic dermal product. 



