40 MOLLUSCA. 



the Cephalophora. It corresponds in position and structure to the 

 organ of Bojanus of Lamellibranchs. It is, however, usually un- 

 paired, and lies near the heart as an elongated triangular sac, with 

 spongy (rarely smooth) walls of a yellowish brown colour. The 

 secretion of the gland consists mainly of hard concrements, which 

 arise in the lining cells, and consist of uric acid, calcareous and 

 ammoniacal salts. It opens near the anus into the mantle cavity, 

 either immediately by a slit capable of being closed, or by a special 

 excretory duct running with the rectum. 



The Gastropoda generally possess, in the roof of the respiratory 

 cavity, a mucous gland, which often pours out an enormous quantity 

 of its secretion through the mantle orifice. The purple gland 

 (Purpura, Murex) lies in the roof of the mantle cavity, near the 

 rectum. It is a long, whitish-yellow glandular mass, the colourless 

 secretion of which, according to the investigations of Lacaze-Duthier, 

 quickly acquires, under the influence of sunlight, a red or violet 

 colour. The secretion of this gland was known to the ancients, and 

 prized by them on account of its permanence. The coloured fluid, 

 which is excreted from pores of the skin of many Opisthobranchs, 

 e.g., Aplysia, must not be confounded with the genuine purple. 



Another gland, whose function is not accurately known, is the 

 pedal gland of Limax and Arion. It extends through the whole 

 length of the foot, and consists of unicellular glands, the delicate 

 ducts of which open into the band-shaped main duct. The latter 

 opens to the exterior between the foot and the head. In many 

 naked Pulmonates (Arion) there is, in addition, a gland at the 

 point of the tail, which secretes considerable quantities of mucus 

 with great rapidity. 



Generative Organs. Some of the Gastropoda are dioacious, some 

 are hermaphrodite. The Pulmonata and Opisthobranchiata are her- 

 maphrodite ', the Prosobranchiata are dioacious. Almost all Gastro- 

 pods lay eggs, usually in strings. Only a few bear living young, 

 which have developed from the fertilised eggs in the uterus. 



The female organs consist of an ovary, oviduct, albumen gland, 

 uterus (dilated and glandular part of the oviduct), vagina, and 

 receptaculum seminis. 



The male organs consist of a testis, a vas deferens with seminal 

 vesicle, a ductus ejaculatorius, and external copulatory organs. 



The hermaphrodite forms are distinguished by the close connection 

 of the male and female generative glands and their ducts ; for not 

 only are the latter in direct communication with each other, 



