180 



INVERTEBRATE MORPHOLOGY. 



also found in Mammalia and in exceptional cases in man. A 

 great uniformity of structure exists throughout all the species, 

 so that they are all referable to a single genus, Echinorhynchus. ' 

 The body (Fig. 90) is cylindrical and as a rule not very long, 

 and a marked distinction from the 

 Nematodes is found in the retractile 

 proboscis {pr) occurring at the anterior 

 end of the body. It is a cylindrical 

 prolongation of the anterior portion of 

 the body and is provided with a number 

 of chitinous hooks by means of which 

 it adheres to the intestinal wall of its 

 host. The proboscis may be invagi- 

 nated into a double-walled muscular 

 proboscis-sheath by whose contraction 

 it may again be protruded, a strong 

 retractor muscle, extending from the 

 tip of the proboscis to the base of the 

 sheath, serving for the invagination ; 

 and from the base of the sheath re- 

 tractor muscles [rm) pass to the body- 

 walls and serve to hold the sheath in 

 position. No traces of a digestive tract 

 occur. 



The body is covered upon the out- 

 side by a thick cuticle secreted by the 

 subjacent hypodermis, which is a rather 

 Fig. 90.— Malb Ecldnorhyn- thick laj'er consisting of a protoplasmic 

 c7jms (after leuokabt). matrix in which nuclei are scattered 

 but in which no cell-outlines are to be 

 distinguished. Beneath the cuticle the 

 matrix has a fibrillar character, and 

 near its inner surface it is hollowed out 

 into a network of anastomosing canals 

 of which mention will be made later. 

 Beneath the hypodermis lies a basement-membrane within 

 which are two layers of muscle-cells, having the same epi- 

 thelio-museular character as those of the Nematodes, the 

 fibres of the external layer having a circular direction, while 



g = glands. 



I = lemniscus 



p = penis. 

 pg = proboscis ganglion. 

 po' = proboscis. 

 rm = retractor muscle. 

 I = testis. 



