LAME^LIBRANCHIATA. 23 



In own spongy tissue, which is covered with a closely ciliated layer 

 of cells, from which concrements containing calcareous matter and 

 uric acid (also guanin) are excreted. The simple duct often receives 

 the duct of the generative apparatus, or the two organs open 

 together on a common papilla on either side. In the Siphoniata, on 

 the other hand, the renal and generative openings are almost always 

 separate. 



Generative organs. The Lamellibranchs are, with a few 

 exceptions (the genera, Cyclas, Pecten, Ostrea, Clavagella, Pandora), , 

 dioecious. Both kinds of sexual organs lie amongst the viscera, and 

 have the form of lobed or racemose glands, which are placed near the 

 liver, surround the windings of the intestine, and extend into the 

 base of the foot. The testis and ovary can usually be distinguished 

 from one another with the unaided eye by their colour ; the ovary 

 being red in consequence of the colour of the ova; the sperm, on 

 the contrary, is milk-white or yellow. The openings of the ducts 

 are placed light and left near the base of the foot. The form, 

 position and opening are exactly the same in the hermaphrodite 

 glands, in which the male and female follicles may be separate and 

 open separately (Pandora) or together (Pecten, Olavagetta, Cyclas) ; 

 or the same follicles may function sometimes as ovary and sometimes 

 as testis (Ostrea, Cardium norwegicum). In the dioecious forms, 

 the male an<^ female animals may differ in the shape of the shell, as 

 is the case in the fresh water Unionidce. Here the outer gills of the 

 female are used for the reception of the eggs (brood pouch), and 

 the shell is more arched. Hermaphrodite individuals are met with 

 among the fresh water mussels, both in Unio and in Anodonta. 

 The fertilization of the eggs is probably usually effected in the 

 mantle or branchial cavity of the female, 



But few Lamellibranchs are viviparous. The fertilized eggs, 

 however, almost always remain for some time between the valves of 

 the shell, or pass into the branchial leaflets, where they undergo the 

 early processes of embryonic development under the protection of 

 the mother. This care of the brood is especially conspicuous 

 in the freshwater forms; in the Unionidce the eggs pass into the 

 great longitudinal canal of the external gill, whence they are dis- 

 tributed into the gill spaces, which become enormously widened and 

 modified into peculiar brood-pouches. When these brood-pouches 

 are emptied the contents are expelled through the great longitudinal 

 canal as a mass of eggs, united together by mucus and containing 

 ciliated embryos, or as a continuous string of eggs. 



