174 



INVERTEBRATE MORPHOLOGY. 



they are found in each of the two orders into which the class 

 may be divided. 



-ph 



1. Order Eunematoda. 



This order contains the majority of the Nematoda, and 

 all its members are furnished with a mouth and anus and a 

 functional digestive tract. The mouth is in 

 some cases at the bottom of a funnel-like 

 depression which may be armed with spines, 

 special developments of the cuticula which 

 covers the body. This is throughout cylin- 

 drical in shape, except that in the males of 

 some species it expands at the posterior ex- 

 tremity into a relatively large funnel-shaped 

 structure with thin walls, the bursa (Fig. 87), 

 at the bottom of which lies the opening of the 

 cloaca, a cavity into which the intestine and 

 the male reproductive organ open. Beneath 

 the ringed cuticle lies the ectoderm (liypo- 

 dermis] which secretes it, and beneath this 

 the muscular layer Avhich consists only of 

 longitudinal muscle-fibres, differentiations of 

 the outer ends of large cells whose undiffer- 

 entiated inner ends project into the coelom, so 

 as almost to obliterate it in some cases. The 

 muscle-fibres do not, however, form a com- 

 plete continuous sheath surrounding the 

 coelom, but are interrupted along four longi- 

 tudinal lines, two lateral, one dorsal, and 

 one ventral (Fig. 88). The coelom contains 

 the intestine and reproductive organs, and 

 is peculiar in that it is not bounded by a 

 limiting cellular membrane or peritoneal 

 lining, being simply a space comparable to 

 the coelomio cavities of the Rhabdoccela or 

 the blood-sinuses of the Nemerteans. 



The digestive tract is a straight tube traversing the body 

 from one extremit} r to the other, opening posteriorly in the 



W. 



FIG. 87. Ascaris 



(after LEUCKAUT). 

 i =- iutestiue. 

 ph pharynx. 

 sp = spicules. 

 te = testis. 



