CHAPTER VII 

 THE INTESTINE OR MESENTERON 



A. GENERAL MORPHOLOGY 



The intestine of nematodes is a tube the wall of which is 

 composed of epithelial cells. Its gross morphologj' does not 

 vary markedly in different groups of nematodes. Usually it 

 is a simple, more or less straight tube accommodating itself 

 to the reproductive organs and space in the body cavity. 



Subdivisions of Intestine. — The intestine may be divided 

 into three regions: the anterior part or ventricular region; mid- 

 region or intestine proper; and the posterior part or prerectal 

 region. The ventricular and prerectal regions commonly differ 

 from the mid-region in the height of the cells and shape of 

 the lumen. Usually, there is also some difference in the type 

 of cell inclusions present in these regions. When a region is 

 quite definitely differentiated from the remainder of the intes- 

 tine it is herein termed either a ventriculus (anterior) or pre- 

 rectum (posterior) while an adjectival usage is retained when 

 the differentiation is not marlced. 



Appendages. — Two types of cecae or diverticulae occur in 

 the ventricular region, one directed anteriorly, the other pos- 

 teriorly. The first t.vpe is by far the more common, occurring 

 in various degrees of development in memliers of several groups 

 in the Phasniidia. Only one free-living nematode is known 

 with such a structure, namely Eliabdilis c.nlinrlrica, and in this 

 instance the cecum is ver.y small, scarcely a third of the in- 

 testinal diameter in length. Likewise but one member of the 

 Strongylina, Grammocephahis (Ancylostoniatidae) has been 

 described as po.ssessing a short intestinal cecum and but 

 a few representatives of the order Spirurida (Dacnitis spp. and 

 Dichelynf spp.) have such structures. The cecae in these forms 

 are quite small. Development of ventricular cecae is most 

 common in the Ascaridoidca. sporadicall,v occurring in such 

 forms as Cimtracaccnm, Avfiusticdrriim and .tiiipUfaiciim (As- 

 carididae, Anisakinae). In the last mentioned forms the 

 cecum may be very large (Fig. 97 G-M), extending far beyond 

 the base of the esophagus, even to the nerve ring. A posterior- 

 ly directed cecum (Fig. 09D) is known to exist only in females 

 of the genus Leiili/nema (Thelastomatidae). 



No satisfactory explanation of the intestinal cecum develop- 

 ment in nematodes has yet been made. The sporadic occurrence 

 of this structure does not seem to be correlated with feeding 

 habits. Phylogeny throws no light on the subject, for closely 

 related forms may differ in this respect. Though the cecum is 

 always a development of the vrnlriciilar region its cells do 

 not differ cytologically from the remainder of this region, 

 indicating no functional specialization (Fig. 98D). 



TjAyers. — The intestinal wall consists of a single layer of 

 ej)ithelial cells which usually bear on their internal surface a 

 bncillary layer (Stabchensaum, liordcur en brousse), and some 

 times a distinct siibbacillary Inyrr (Deckschicht) is apparent. 

 The external surface of the cells may be quite naked, or it may 

 be covered by a distinct basal lameUa. a " musculari? mnco- 

 sae," and a mesenierml mrmbranr. One or more of these 

 coverings may be present or the.v all may appear to be absent. 



Sometimes the protojilasm of the epitlielial cells is divisible 

 into distinct zones. The ecto])lasmie zone is a la.ver of dense 

 cytoplasm bordering the sides of the cell: when definitely 

 thickened on the side of the cell facing the lumen it is called 

 a plasma cap (Fig. 10.3.T3, Z.S"). The remainder of the cell is 

 termed the endoplasm ; it contains the nucleus, cell inclusions 

 and sometimes other structures such as plasma strings (Fig. 

 in.'i.lri), basioplasm (Fig. Kl.'i.Ki), basal fibrillae, etc. 



Thr BaciUary and Siibbarilhtry I.ayrrx. — The baeillary layer 

 consists of an internal border apjiearing to be made up of 

 fine rods or "cilia" bene;ith which one often finds a sub- 

 l)acillary layer (Deckschicht) which stains with iron-hematoxy- 

 lin. The first layer varies markedly in ai)pearance, the bacilli 

 sometimes l)eing rather large and well sejjarated, sometimes 

 compact, sometimes fine and hair like. Tender ordinary circum- 

 stances the baeillary layer has a compact .appearance but the 

 cleuients h;ivc been seen (piite discretely in living si)eciniens of 

 Khabililif: slrongyloiiles. In general, the baeillary and sub- 

 bacillary layers are most highly develojied (thickest) in repre- 

 sentatives of the Strongylina (Fig. 10i!K) and impart a char- 

 acteristic appearance which one is not likely to confuse with 



that of other nematode groups. S )metinies due to fixation the 

 b.'icillary layer may have pulled away from the epithelium, 

 giving the ajipearance of a ])erilrophic membrane. Like the 

 liacillary la.ver of other organisms it is digested by proteolytic 

 enzymes and is therefore non-chitinous. (The peritrophic 

 membrane of arthropods is chitin). 



Somewhat extended discussions of the significance of the 

 baeillary layer have been given in the past. Since this layer 

 is not i)eculiar to the Nematoda, but occurs in the intestine of 

 various groups of worms as well as in arthropods and verte- 

 brates, conclusions based on the study of nematodes alone 

 could scarcely be considered valid. The several viewpoints 

 expressed have been as follows: (1) The bacillar.v layer is a 

 development of minute tubes which aid in resorption or ex- 



A-C — Third stage larval strongyles showing oligocyty. ( .\ — Gyalo- 

 repfialiis capUutun ; B — Cylicoeercus gohli; C — Cylicoceycus ctttiiuUus) . 

 1> — hfidiiiiema appendicutatiim (Adult feTiiale showing cecum). A-C, 

 after I.ucker, IHIifi, Pror. Helm. .Snr. W.Tsh . v. 3(1). 1), after Chit- 

 wood, l!t:i2, /tsi-hr. I'araBitenk., v. .t(1). 



I no 



