PTEROPODA AND GASTROPODA. 541 



by a rudimental, or fully developed shell, form the order Pulmonata. 

 In all the foregoing orders of Gastropods the male and female organs 

 of generation are associated in the same individual. In the re- 

 mainder of the class the sexes are distinct. A small order of marine 

 Gastropods, including the Fissurella and Haliotis^ which have their 

 pectinated branchiae protected by a wide shield- shaped shell, is 

 called Scutibranchiata. Another small group in which similar 

 branchiae are protected, with the entire body, by a tubular shell, is 

 called Tuhulihranchiata. In the last and highest organised order, 

 called Pecti?iibranchiata, from the comb-shaped gills which have a 

 special cavity at the fore part of the back, the males are provided with 

 an intromittent organ. 



The soft parts of the Gastropods are immediately invested by a 

 soft inarticulated lubricous integument, forming, in most, a sub- 

 circular fold {Jig. 199, c) about the neck, behind which it is dilated 



into a sac (ib. d) containing a portion 

 of the viscera, which fold and sac are 

 called the "mantle." In the skin 

 may be recognised an epidermal, a 

 pigmental, and a dermal layer, the 

 latter being of a musculo-cellular 

 structure and highly contractile. 

 '^^^^^- The epiderm is ciliated over 



nearly the whole body in the aquatic Gastropods, but only in certain 

 spots in the terrestrial species. The white striae at the sides of the 

 neck and foot of Helix are composed of short, thickly-set, calcareous 

 needles: the entire skin of Polycera and some other Nudibranchiates 

 is studded with analogous ramified spiculae. 



The shell results from the metamorphosis and calcification of cells 

 deposited in layers beneath the epidermis, in the situation of the 

 rete-mucosum in the human integument. In Limax and Clausilia 

 the first trace of the rudimental shell is in the form of crystals of 

 carbonate of lime in the substance of the mantle.* Its formation in 

 the univalve Gastropods commences in the embryo, and the first- 

 formed part is called the nucleus of the shell ; the succeeding layers 

 are not, however, formed around this, but are added to the inner 

 surface of the circumference of the previously formed parts ; and the 

 proportions in which the new-formed layers extend beyond their 

 predecessors determine the figure of the future shell. In some 

 Gastropods, at certain seasons, the margin of the mantle, in which 

 the shell-forming process is most active, extends outwards at an 



* CCCXLI. p. 372. 



