TYPE ARACHNIDA. 461 



scribed here, and are the orders of the Pentastomidce, the 

 Pycnogonida, and the Tardigrada. 



Order Pentastomidae. 



The Pentastomidse are all parasitic, living in the adult 

 stage in the lungs or nasal cavities of various animals, one 

 species, Pentastomum tcenioides, occurring in the nasal cavities 

 or sinuses of dogs and wolves, while several species have been 

 found in the lungs of different species of snakes (Fig. 211). 

 They are all elongated worrnlike forms, some- 

 times slightly flattened and usually distinctly 

 annulated, the annuli, however, not repre- 

 senting a metamerism. The anterior end of 

 the body is rounded and bears on the ventral 

 surface the mouth, upon each side of which 

 is situated a pair of strongly-recurved hooks 

 (h] supplied with special muscles and serving 

 for the attachment of the animal to the tissues 

 of the host. "With the exception of these 

 hooks no appendages are present. 



The body is covered by a cuticle secreted 

 by the ectodermal cells (hypoclermis), be- 

 neath which lies a layer of circular niuscle- 

 fibres, and beneath these again a layer of 

 longitudinal muscles. The coelom is ample 

 and is traversed by dorso-ventral muscle- 

 bands, which divide it into a central com- 

 partment containing the various organs, sus- IG ' ~ i. rena- 



- 



pended by mesenteries, and two lateral ones. lum p EMALE (after 

 There is no heart or circulatory apparatus, SPENCER). 



and tracheae or other respiratory organs are r nooks. 



... go = genital orifice. 



also wanting. 



The digestive tract is a straight tube extending through 

 the body from the mouth to the terminal anus, giving off no 

 lateral diverticula throughout its course. The nervous sys- 

 tem (Fig. 212, no] consists of a ganglionic mass lying below 

 the oesophagus, a comparatively small commissural ring 

 passing round that portion of the digestive tract, without, 



