ACANTHOCEPHALA 475 



D. ACANTHOCEPHALA, Rud. 



GUTLESS, nematode-like worms that carry at their anterior end a retractile 

 rostrum beset with hooks. In their adult condition they only live in vertebrate 

 animals. During their larval stage they are often parasitic in invertebrate animals. 



The Acanthocephala are elongated cylindrical worms, with a rounded posterior 

 end. In some species an annulation is distinctly recognizable ; they are, however, 

 not segmented. The size varies according to the species, between about $ to lomm. 

 and 40 to 50 cm. ; in general, however, there is a preponderance of the small species. 

 The sexes are separate, and the males can easily be distinguished from the females 

 without examination of the genitalia, as the females are both larger and thicker. 



The body wall of Echinorhynchus is limited by a thin cuticle, which is attached 

 inwardly to the hypodermis. In only exceptional cases a syncytium with large 

 nuclei, even in the adult condition, is represented by the hypodermis ; and in it fibre 

 systems, the elements of which run in layers in various directions, appear, and it is 

 only towards the interior from these strata of fibres that the nuclei of the hypodermis 

 are found. As a rule, these fibres, at all events the radiary fibres, are regarded as 

 muscles. Hamann desciibes them as elastic fibres, which lie in a viscid gelatinous 

 connective substance (transformed protoplasm ?) ; a lacune system filled with a 

 granular fluid, the central part of which are two longitudinal lacunes lying at the 

 sides, also belongs to the cutaneous strata, as do the so-called lemnisci, two short, 

 flat organs suspended in the body cavity, and the pedicles of which are attached 

 anteriorly at the border between the rostrum and body ; their structure as well as 

 their origin permit them to be traced to the skin (fig. 348A). 



Finally, inwardly below the skin there follows a layer of annular, and after these 

 a layer of longitudinal muscles, the structure cells of which remain present in the 

 residues, carrying nuclei. The motor apparatus of the rostrum, the sheath of the 

 rostrum, and the lemnisci also belong to the muscular system. The rostrum 

 represents a finger-shaped hollow process of the cutaneous layer ; but, according 

 to Hamann, it originates from the entoderm and passes through the skin secondarily. 

 It is covered by a thin cuticle, and as a rule contains a large number of regularly 

 placed chitinous hooks that adjoin a granular formation tissue. From the base of 

 the rostrum springs a tubular hollow muscle extending into the body cavity ; this 

 is the RECEPTACULUM PROBOSCIDIS, from the base of which again bundles of 

 longitudinal muscles originate, which pass along its axis and that of the rostrum 

 itself, and are inserted at the inner surface of its anterior end (RETRACTOR 

 PROBOSCIDIS). These muscles when they contract invaginate the proboscis and 

 draw it into the receptaculum ; when reversed they act again as PROTRUSOR 

 PROBOSCIDIS. The whole of the anterior body, however, can be invaginated, and 

 for this purpose there is a muscle that originates from the body wall at a variable 

 distance back, and which is joined to the receptaculum (RETRACTOR RECEPTACULI) ; 

 there is also a bell-shaped muscle which springs from the body wall behind the 

 lemnisci in rings, and passes forward to the spot of attachment of the lemnisci. 



The nervous system consists of a cluster of ganglia situated at the base of the 

 rostrum, from which three nerves pass towards the front and two towards the back. 

 No sensory organs are known. 



The excretory organs, according to Kaiser, lie at the upper border of the ductus 

 ejaculatorius in the male and at the so-called bell in the female. Here they 

 jo* 



