LAMELLIBRANCHIATA. 341 



dorso-ventral lines, each fold including a certain number of filaments. The 

 filament at the re-entering angle of each fold is stronger than the others. 



Summary and additional details of gill-structure. The successive filaments 

 of the same row are united () by the cilia of the lateral cells (Nuculidac, 

 Solcnomyidae, Anomiidae) ; (b) by the cilia of the ciliated disks of the ciliary 

 junctions (Arcidae, Trigoniidae, Mytllidae, Avicula, Pecten, and Mcleagrina) ; 

 by vascular interfilamentar concrescences (Lima, Ostrea, Pinna, and all Eula- 

 mellibranchs). Inter lamellar concrescences, i.e. fusions between the ascending 

 and descending limbs of the same filament, or between the lamellae of the same 

 gill, are absent in Anomiidae, Arcidae, Trigoniidae ; they are present in a non- 

 vascular form (i.e. consist only of epithelium and connective tissue) in Mytilidae, 

 Pectinidae, and in a vascular form in Pseudolamellibranchs except Pectinidae, 

 and in Eulamellibranchs. Finally there are forms like Lima with non-vascular 

 interlamellar concrescences and vascular interfilamentar junctions. 



Concrescence of the gills with the mantle and with those of the opposite side, 

 is absent in Nuculidae, Solenomyidac, Arcidae, Trigoniidae, Pectinidae. In 

 Anomia there is concrescence between the two branchiae, but none with the 

 mantle. In all other Lamellibranchs they are fused to the mantle by the 

 ascending limb of the outer filaments, and to their fellows across the middle 

 line by the ascending limb of their inner filaments behind the foot. 



Excretory system and pericardium. The pericardium is a dorsal 

 median chamber enclosing the heart (except in Anomia). There are 

 two nephridia the so-called Oryans of Bojaims. These usually have 

 the form of twisted tubes, more or less dilated in certain parts, and 

 opening at one end into the pericardium and at the other to the 

 exterior on the ventral surface of the body on each side of the foot. 

 The part of the tube which opens into the pericardium is generally 

 lined with yellow or dark-coloured glandular tissue, which secretes 

 concrements containing calcareous matter, uric acid, and guanin. The 

 kidneys of the two sides communicate in some forms. In Osfrea the 

 glandular part of the kidneys is a branched gland ramifying on the 

 surface of the visceral mass. 



The pericardial glands are differentiations of the epithelial lining 

 of the pericardium. They may be placed on the auricles, to which 

 they impart a yellow colour, or near the auricles at the anterior end 

 of the pericardium. 



Generative organs. The sexes are, with a feAv exceptions (some 

 species of Pecten, Ostrea, Cardium, and the genera Cydas, Pisidium, 

 Porornya, Eniovalva), separate, and except in some species of Unio 

 do not present external differences. The generative glands 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, and in some forms 

 into the mantle lobes (Mytihis). The testis and ovary can often be 



