FRESH-WATER MUSSEL. 129 



thinness. At its anterior edge a black bristle has been passed into 

 the mouth. A ridge or lip above the mouth, and another below it, are 

 prolonged respectively into the right and left pairs of labial tentacles, 

 which project like wings in this specimen. Behind these tentacles, at 

 the sides of the visceral mass, and between it and the mantle-folds, are 

 the branchiae or gills. There are two gills or ctenidia on each side. 

 Each gill is composed of two lamellae, an outer and an inner. The inner 

 lamella of the inner gill is attached most anteriorly to the side of the 

 visceral mass above the outer tentacle. It is then free for a part of its 

 course, but posteriorly unites with its fellow, as may be seen in this 

 specimen. The junction of these two inner lamellae inter se separates 

 the infra- from the supra-branchial chamber, and is the cause in part of 

 the posterior indirect union of the mantle-folds (see also Preparation 26). 

 The outer lamella of the outer gill is fused in its whole extent to the 

 mantle, and it assists where the ctenidial axis becomes free posteriorly in 

 dividing the supra- and infra-branchial chambers, and in causing the 

 indirect union of the mantle-folds. 



The internal surface of the mantle is covered with ciliated epithelium, and 

 the plasma-cells (p. 115) of its connective tissue contain glycogen. In many forms 

 its ventral edges coalesce or ' concresce ' leaving an aperture for the foot ; or the 

 process may be carried even further when that organ is aborted, e. g. in Asper- 

 gillum, and then the only entrance to the mantle cavity is through the inhalent 

 siphon. When an Anodonta is removed from its shell, there is seen (best in a 

 fresh specimen) a reddish streak running from near the anterior to near the pos- 

 terior adductor. This streak is the red-brown organ of Keber, the pericardial 

 gland of Grobben. According to the latter, it consists of a series of caeca com- 

 municating with the pericardial cavity and lined by a continuation of its epithe- 

 lium. The homologous structure in Cephalopoda forms an appendix to the 

 branchial heart, is similarly formed, and a portion of its epithelium, that in the 

 peripheral caeca, is excretory in structure. 



The foot is in some Lamellibranchiata aborted, e. g. Oysters. In others it is 

 very small, and its shape varies much within the limits of the class. The ventral 

 edge has, according to Griesbach, in the Anodonta three pores of fair size, one 

 placed anteriorly, two somewhat posteriorly. He states that water finds its way 

 through them to the vascular lacunae, a fact disputed by other authorities. Similar 

 pores exist in other Lamellibranchiata and Mollusca (?). Water has been sup- 

 posed to find its way into the blood-system in one of three ways through the 

 nephridium into the pericardium, through special pori aquiferi, and through inter- 

 cellular passages between the ectoderm cells or epidermis. But the great dis- 

 tension of the foot in Anodonta^ when protruded from the shell for the purposes 

 of locomotion, is due apparently to the action of a circular muscle surrounding the 

 vein which conveys the blood from the foot on each side of the body to the 

 median infra-cardiac sinus. By this means the return of the blood is prevented 

 whilst the heart at the same time continues to drive blood into the foot. Certain 



K 



