1/4 NEMATHELMINTHES chap. 



been obtained from the thoracic cavity of a prawn, Pcdaemonetes, 1 

 which has thus some claim to be regarded as the host of this 

 species, but nothing is known about its early life-history. 



Sub-Order III. Acanthocephala. 



The Acanthocephala, which form the third class of the Nema- 

 thelminthes, consists of but few genera; there are, however, 

 numerous species of very different size, varying from 10 to 6 5 

 cm. long in the female Gigantorhynchus (Uchinorhynchus) gigas, 

 to quite minute forms a few millimetres in length. The adult 

 stage occurs in the alimentary canal of Vertebrates, as a rule 

 in those which live in, or frequent water ; the larvae are found 

 in the bodies of certain Invertebrates, very frequently small 

 Crustacea. 



Anatomy. The body of the mature forms can usually be divided 

 into three sections the proboscis, the neck, 

 and the trunk, but the middle region is not 

 always discernible. The proboscis is armed 

 with rings of hooks (Fig. 93) arranged in 

 longitudinal rows ; they are usually of two 

 kinds, but in E. proteus of three. They have 

 a certain specific value, but not much stress 

 can be laid on the number of rings, e.g. in 

 E. angustatus the number varies from eight 

 FlG - ' 9 ;; 7T T 7 s f cim * ns to twenty-four. The recurved hooks serve to 



of bchmorhynchus pro- J 



tens Westrumb., with fasten the parasite very firmly to the tissues 

 t"y m of ^ the host. The proboscis is hollow and 

 the intestine of a Pike, retractile ; it can be withdrawn into the 



Magnified with a lens. , , , , , -. . 



(From Hamann.) body by means ol muscles attached inter- 



nally to its tip. It does not, however, pass 

 straight into the body-cavity, but is retracted into a special 

 cavity the proboscis sheath with a double muscular wall. 

 The proboscis sheath may perhaps be looked upon as a septum, 

 such as is found in some of the Nematomorpha, dividing the 

 body-cavity into two parts. It is inserted into the body-wall 

 at the junction of the neck and trunk or of the proboscis and 

 trunk. In addition to the muscles which withdraw the pro- 

 boscis into its sheath, there are two retractors running from the 

 1 H. B. Ward, P. Amer. Ac. new ser. vol. xix. 1892, p. 260. 



