130 DESCRIPTIONS OF PREPARATIONS. 



regions of the mantle appear to act as blood-reservoirs from which a store of 

 blood may be drawn under these circumstances (Fleischmann). It may be added 

 that in Solen (Ceratisoleri) legumen, where some of the blood-corpuscles are tinged 

 with haemoglobin, these corpuscles do not escape from the blood even when the 

 animal is greatly irritated and consequently strongly contracted. Any direct pas- 

 sage of water from without into the blood, or escape of blood, must consequently 

 be regarded as an extremely doubtful occurrence. In some specimens of An- 

 odonta there exists a small pit on the posterior margin of the foot. This, accord- 

 ing to Carriere, is a remnant of the byssus gland, which is so well developed in 

 Dreissena among fresh-water Bivalves. The young Anodonta (= Glochidiuni] has 

 a byssus filament, but the gland which secretes it disappears, and the true byssus 

 gland appears later. The filament referred to is adhesive, and clings to anything 

 which it touches. 



The labial tentacles are vascular, richly supplied with nerves, and ciliated. 

 Their opposed surfaces are covered with fine parallel ridges. The upper tentacles 

 appear to rise in part from the mantle, the lower from the sides of the visceral 

 mass, but the two tentacles of the same side undergo partial concrescence of their 

 surfaces, and the furrow between them does not extend back between the mantle 

 and visceral mass. In some instances they are of very large size. Embryology 

 does not favour an homology with gills. Professor Ray Lankester however has 

 suggested that they with the gills are homologues of the prae- and post-oral 

 ciliated bands of the Echinoid and Ophiurid larva, Pluteus, or the Tornaria larva 

 of Balanoglossus. 



The gills require careful examination. In Anodonta they appear to be pro- 

 foundly modified from their original structure. Each gill consists of two lamellae, 

 an outer and an inner. The inner lamella of the outer gill and the outer lamella of 

 the inner gill arise close together and along the line between them run the afferent 

 and efferent blood-vessels. This line represents the original (ctenidial) axis of the 

 gill. Posteriorly the axis is free for a short distance, but anteriorly it is part of the 

 under side of the organ of Bojanus and the side of the body. The space between 

 the two lamellae, i. e. outer and inner, of each gill, is the ' interlamellar ' space, and 

 examination shows that it is crossed by numerous ' interlamellar ' junctions. If the 

 surfaces of the lamellae be regarded attentively, they are seen to be perforated by 

 many series of apertures. Hence the lamellae resemble a fenestrated membrane, 

 but may be regarded as composed of a number of parallel vertical filaments united 

 from place to place by ' interfilamentar ' junctions. The latter is the view justified 

 both by a comparison with certain other forms as well as by embryology. Nucula 

 and Yoldia (Arcaced) have each ctenidial axis bearing two series, an outer and inner ? 

 of gill-filaments which are lamellate in shape. In Mytilus and other bivalves they 

 are filamentous, and the filaments of the outer series are folded on themselves, 

 the folded part being external, while the inner filaments are similarly folded, but 

 the folded part is internal. The filaments are also united laterally in Mytilus, Area, 

 &c., by peculiar long cilia into a lamella, the component filaments of which are 

 easily separated. Solid interlamellar junctions are sparingly developed. Further 

 steps in complication are, the development of tubular interfilamentar junctions, e. g. 

 in Dreissena, the union of the reflected portions of the filaments to neighbouring 

 parts and inter se, and the great development of interlamellar junctions. The 



