1/4 NEMATHELMINTHES 



been obtained from the thoracic cavity of a prawn, Palaemonetes, 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 (Echinorhynchus) 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, 



^^P>> and the trunk, but the middle region is not 



^^Mj^ always discernible. The proboscis is armed 



jjt wfy> with rings of hooks (Fig. 93) arranged in 



mFfy longitudinal rows ; they are usually of two 



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



FA a certain specific value, but not much stress 



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



E. angustatus the number varies from eight 



F ' G of 9 c7i^ y nS to twenty-four. The recurved hooks serve to 

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

 ^LTfaX^winf of the host - The proboscis is hollow and 

 the intestine of a Pike, retractile ; it can be withdrawn into the 



Magnified with a lens. -,-,, /. i , i i , 



(From Hamann.) " od j "Y means of 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. Atner. Ac. new ser. vol. xixl 1892, p. 260. 



