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CLASS ACANTHOCEPHALA. 



This small class of entoparasitic Vermes contains the single genus 

 Echinorhynchus with many species very variable in size. The adult worm 

 is found in the alimentary canal of some Vertebrate, the immature in the 

 body of a Non-vertebrate ; e. g. Ech. Proteus of the Pike and other fresh- 

 water fish is found in the body cavity of the Amphipod Crustacean Gam- 

 marus pulex ; E. angiistatus of the Perch in the Isopod Asellus aquaticus. 



The body is divisible into three regions, a proboscis, neck, and body 

 proper, which is cylindrical, and when contracted, transversely wrinkled. 

 The proboscis carries a variable number of chitinoid recurved hooks, 

 arranged in longitudinal rows, and is plunged into the tissues of the host. 

 The body-walls consist of an external delicate cuticula, a striated sub- 

 cuticula, a layer of circular and most internally of longitudinal muscle-cells. 

 The subcuticula has an outer layer composed of a complicated tissue of 

 circular and longitudinal fibres and an inner layer of radial fibres, amid 

 which lies a system of canals. The subcuticula of the body proper, and 

 consequently its canal system, are separated from the corresponding 

 structures in the neck by a fold of cuticula. Two remarkable semilunar 

 masses of subcuticula, the lemnisci, depend from the neck into the body 

 cavity. They vary in length in difTerent species. The canal system of the 

 body proper consists of two longitudinal vessels, right and left in most 

 instances, dorsal and ventral in Ech. clavaeceps, with lateral vessels which 

 branch and anastomose. There is a circular canal at the base of the 

 neck connected with an irregular network, but in the proboscis longi- 

 tudinal vessels alone are found, one between each series of hooks, united 

 laterally between each pair of hooks. The circular vessel also sends into 

 each lemniscus two principal vessels, each of which divides into two with 

 a number of secondary branches. There is no opening into the system of 

 canals. Their contents are a granular liquid. The granules appear to 

 be of a fatty nature, and are orange coloured, or in the lemnisci of Ech. 

 Proteus brownish-yellow. Nuclei of large size are also found in the canals 

 as well as in the intervening fibres, and differ in shape, &c. in the body 

 proper and the neck. The muscular layers of the body are continued into 

 the neck, and the longitudinal layer forms a sheath for each lemniscus. A 

 longitudinal muscle layer is absent in the proboscis. The muscular tissue 

 is peculiar and somewhat like that of Nematoda, the striated fibrillar 

 substance being developed only on the outer surface of the cell. 



A double circular layer of muscular cells forms a proboscis sheath, 

 which hangs down towards the coelome and is attached to the base of the 

 proboscis. It contains four longitudinal retractor muscles for the proboscis, 

 by means of which that structure can be invaginated. These retractors are 



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