INTEGUMENT OF MOLLUSCA. 329 



large number of thein, so that they may form a veritable calcareous 

 network, as in Doris, Polycera, etc., the vai-ious species of which are 

 distinguished by the special manner in which the various calcareous 

 rods are grouped or arranged, as well as by the way in which they 

 are formed. 



§ 256. 



The glands are differentiations of the epidermis; they partly 

 resemble the structures found in Vermes (unicellular glands) . When 

 simplest, these organs are modifications of epidermal cells, which 

 are placed between other cells, but are distinguished from them by 

 having finely-granulated contents and a mouth (goblet-cells). They 

 are found in the Lamellibranchiata as well as in the Gastropoda. In 

 the Cephalopoda they are more commonly arranged in groups, and 

 their blind ends extend below the level of the epidermis. In the Gras- 

 tropoda — and especially in the terrestrial Pulmonata — they are found 

 to be placed still deeper in the integument. These glands are variously 

 modified in different parts of the body. Those found at the edge of 

 the mantle in the shelled Gastropoda are examples; they secrete 

 a fluid in which calcareous salts are dissolved, while others secrete 

 colouring matters. 



In Aplysia the dermal glands secrete a dark-red fluid. In 

 Murex and Purpura a layer of epithelium, which is placed between 

 the gills and the hind-gut, and in the mantle-cavity, functions as a 

 gland; this layer is formed by large superficially-ciliated cells. 

 Their secretion gives rise to the substance known as Tyrian purple. 



Some Opisthobranchiata (j^jjolidiae) are characterised by the 

 possession of urticating cells, which are placed on the ends of 

 their dorsal papillae. 



The Byssus-gland of the Lamellibranchiata is a more inde- 

 pendent glandular organ of the integument ; when it is formed the 

 foot undergoes certain modifications ; it becomes, that is, a tongue- 

 shaped process grooved on its ventral surface. The groove passes to 

 a depression at the base of the foot, at the bottom of which there is a 

 gland which secretes the so-called " byssus.''^ Pecten, Lima, Area, 

 Tridacna, Malleus, Avicula, Mytilus have an organ of this kind; 

 but it may be considered as generally present, for it is found for a 

 time in the embryos of the Naiades and of Cyclas. Some divisions 

 of the Gastropoda (Helicinse, Limacinse) have also a gland in their 

 foot, which opens anteriorly and below the mouth. A large number 

 of other kinds of glandular organs are also developed from the 

 integument. 



Shells. 



§ 257. 



The tegumentary investment is of special importance, for it 

 secretes firm substances, which are laid down in layers, and which 



