THE PARASITES OF THE PEARL OYSTER. 97 



in optical section. Immediately behind this is a thin-walled vesicle representing 

 the oesophagus. From this issues the long unbranched, thick-walled digestive 

 caecum (d.c). 



The excretory system is more highly specialized than in the two Distomids already 

 described. As in them, it consists of a lateral trunk system opening behind into a 

 contractile vesicle. In place, however, of arising in the pharyngeal region and passing 

 backwards direct to the contractile vesicle, each trunk is doubled and consists of a 

 proximal and a distal section. The proximal, which receives numerous branch feeders 

 in its course, arises in the posterior portion of the body, close to the termination of 

 the digestive caecum. Thence it runs forwards to the anterior end of the pharynx, 

 where it loops and turns upon itself, passing backwards over nearly the same course 

 as it came. As it goes it coils around the primary or proximal portion. Both 

 divisions are richly ciliated, a current is observable passing forwards in the cavity of 

 the proximal limb, towards the hinder end in the other, or the distal limb. Another 

 (third) tube or band is very faintly visible running longitudinally. Possibly it is a 

 sexual duct. 



The only specimen which could be spared to the knife was immature. There is a 

 median aperture just between the pharynx and the anterior end of the foot. The 

 penis is well-marked, the testis is single and so is the ovary, both lie in a mass of 

 parenchyma which is separated above from the mass in which the alimentary canal 

 lies, and below from the foot by two sheets of muscles. No vitellaria, uterus, or 

 L-VTJREr's canal were distinguishable. 



A pale smoky yellow tint suffuses the entire body, saving in the tissues lying 

 dorsal to the tube-feet, where a warm brick-red tint is distinctive. The tube-feet 

 appear to be colourless. 



When one of these Aspidogasters is extracted alive from the pericardium, it 

 exhibits an active and restless disposition, crawling freely about in a watch-glass. Its 

 habit when thus isolated is to attach itself firmly by the suckers of the pedal disc and 

 to wave the long neck-like anterior region from side to side, upwards and downwards, 

 after the manner of a leech scenting or searching for prey, swaying to the extreme 

 right, back almost to the posterior end of the body, then swiftly swinging round it 

 repeats the search upon the left side, then forwards and above (Plate IV., figs. 61 

 and 69). 



The manner of progression resembles that of a leech. Making adhesion over the 

 whole of its ventral sucking disc, the mouth rises from the surface to which it has till 

 now adhered ; the neck stretches forwards, lengthening to the utmost, it curves 

 downwards, the lips part, and the oral sucker makes a fresh adhesion (Plate IV., 

 figs. 69). The posterior portion of the body is next drawn forwards, the anterior 

 suckers of the disc remaining fixed the while ; then, where the posterior region is 

 well shortened, the whole of the ventral disc is freed and drawn along to the point 

 where the oral sucker is fixed, when the disc re-attaches first at the anterior end and 



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