THE PARASITES OF THE PEARL OYSTER. 101 



head end are (i rows of about 50 hooks about 0031 niillim. long. The head-lappets 

 are rounded behind and abut on one another in the lateral line. There are 6 lips 

 on the head. The oesophagus is 4-5 of the entire length, a id the conieally pointed 

 (ail end is :> ' 4 of the total length. Four tubes lie beneath the anterior end of the 

 oesophagus; these are 2 - 05 millinis. in length, and, as is characteristic for the genus 

 Cheir acanthus, these are shorter than the (esophagus and show externally a layer 

 of spiral muscles. The larvas were found encysted in the adductor muscle. 



The adult of these forms is in all probability the Cheiracanthus uncinatus, 

 described and figured by Molix under the name Echinocephaliis uncinatus. It is 

 found in the alimentary canal of Trygon pastinaca and of Trygon brucco. 



This second Nematode infesting the pearl oyster is a robust species, readily 

 distinguishable at sight from the Ascarid, by its comparative shortness and the sub- 

 globular enlargement of the cephalic extremity, which is armed with 6 concentric 

 rings of backwardly-directed spines (Plate III, figs. 41, 44, 45). This globular 

 cephalic inflation is characteristic, but at times it appears in a deflated condition, as 

 shown in Plate III., fig. 45, when the form becomes that of a truncated cone. 



To the naked eye it appears when lying in its cyst of a faint pinkish tint, under 

 magnification the alimentary canal is seen to be of a dirty pale-yellow hue. The 

 colourless transparent tissue lining the body wall has a distinct areolar appearance, 

 due to the presence of large saccate cells. The body is smooth, with no trace of 

 annulation. 



This Nematode favours exclusively as its habitat the substance of the adductor 

 muscle, lying coiled up therein in an ovoid cyst. Plate III., fig. 47, shows some of the 

 usual regions in the muscle where it is found. Its occurrence is strictly limited to 

 this particular organ, but occasionally it would appear to become freed from its 

 envelope and to leave the muscle, as instances occurred of this species being entombed 

 in the nacreous lining of the shell. In several cases the coverino- film of nacre 

 obscured scarcely anything of the outer form of the worm's body, the globular head 

 and curved and pointed tail being especially conspicuous. 



Later stages have been found by one of us in the trigger fishes Balistes mitis 

 and B. xtellatus, both pearl oyster-eating species, as proved by the presence of the 

 shell fragments in the stomach. 



This spinous-headed Nematode is found both in the alimentary canal and in the 

 visceral cavity of Balistes. In the latter case, where it is much the more frequently 

 found, it burrows in the peritoneal membranes and adjacent connective-tissue. 

 Judging from this habitat, it would appear to use its spine-armed cephalic extremity 

 as a burrowino; organ. 



It is most probable that the adult also lives in some species of Trygon which are 

 known to feed on file fish, and also on pearl oysters. 



Statistics and Details. In all the cases noted below the Nematode was found in 

 the adductor muscle one Nematode in each case (Plate III., fig. 47). 



