32 



Vena cava receiving the blood of the body which passes through the organ of Bojanus to the 

 gills. 



( Transparent non glandular portion or vestibule communicating with 

 ORGAN OF exterior and with the 



BOJANUS ) Brownish glandular portion communicating with pericardium by two 



(. oval openings (see fig. 1, b). 

 RESPIRATORY SYSTEM 



FIG. G, a. GENERAL STRUCTURE OP GILL, section through line ab in diagram 1 

 Inner and outer lamellae. 



Partitions between lamellae interlamellar partitions. 



Pairs of chitinous rods in the parallel and vertical gill-filaments, indicated by pairs of dots. 

 FIG. 6, l>. MINUTE STRUCTURE OF GILL, section of lamella parallel to surface in region of chitinous rods 

 Gill-filament strengthened by pair of chitinous rods. 

 Openings between filaments furnished with ciliated epithelium. 

 Partitions between filaments, interfilamentar, consisting of horizontal and obliquely crossed 



fibres. 



MUSCULAR SYSTEM 

 FIG. 7. MUSCLES 



Anterior and posterior adductors, adducting or bringing together the valves. 



Protractor of foot. 



Anterior retractor of foot. 



Posterior retractor of foot. 



Smaller retractors arising near umbo, impressions only shown (fig. 8, a). 



EXOSKELETON 



FIG. 8, b. EXTERIOR or SHELL 



Rounded anterior end, tapering posterior end. 



Umbo or beak on dorsal surface of each valve towards anterior end. 

 Concentric lines of growth. 



Elastic ligament behind umbones, binding the two valves together, and tending to open the 

 valves. The hinge-line is without interlocking processes or teeth, hence the name Auodon 

 (Gr. a, without ; odous, odontos, a tooth). 

 FIG. 8, a. INTERIOR OF SHELL 



Pallial impression indicating attachment of muscular ventral edge of mantle to shell. 



Anterior and posterior adductor impressions. 



Successive adductor impressions, extending from cavity of umbo to each adductor muscle, 



and indicating the travelling downwards of adductor muscles. 

 HISTOLOGY 



FIG. 9, a. Liver invests stomach, and consists of blind tubes lined with epithelial cells. 



b. Muscle composed of elongated nucleated cells resembling those of smooth muscle in form 



(PI. XXIV. fig. 7), but having the structure of striped muscle. 



c. Blood-corpuscles, colourless, nucleated, and amo3boid. 

 DEVELOPMENT 



FIG. 10, a. Eipe ovum with its vitelline membrane forming a short tube at one point with an opening 



or micropyle. 



I, c. Embryo when hatched or Glochidium b enclosed in vitelline membrane, c escaped from it. 

 f Valves of shell triangular (see fig. 6), united at base and incurved at 

 DIFFERENCES \ apex. 

 FROM ADULT 1 Si ng i e adductor muscle. 



v Byssus or beard coming off from foot. 

 CLASSIFICATION 



Sub-Kingdom Mollusca (L. mollis, soft). The body is soft and unsegmented ; possesses a 



shell, and three pair of nervous ganglia. 

 Natural Order Lafnellibranchiata (L. lamella, a plate ; Gr. branchia, a gill), because it has 



a bivalve shell ; a two-lobed mantle ; plate-like gills ; and no distinct head. 

 Genus Anodon'ta (Gr. a, without ; odous, odontos, a tooth). 

 Common Name Fresh-water Mussel. 



