Fig. 341.—Larva of Sciara: s.gl, salivary gland; ur.t, urinary tubes; i intestine; st, stomach; cae cæcal appendages; t, testis.
Its inner surface is thrown up into longitudinal folds, generally twelve in number. These folds shine through the outer walls, and are accordingly indicated in the drawings of Dufour, Graber, and others. The entire cæcum has an external muscular envelope, outside of which are a few isolated longitudinal muscular bands. The folds within are formed mainly by the high cylindrical epithelium which lines the whole interior of the cavity. Tracheæ ramify throughout all the layers outside the epithelium. There are appearances of glandular follicles in the bottom of the spaces between the folds. (Minot.)
Burmeister supposed that these cæca were analogous to the pancreas, and this view has been confirmed by Hoppe Seyler, Krukenberg, Plateau, and others, who claim that the digestive properties of the fluid secreted in them agrees with the pancreatic fluid of vertebrates.
Fig. 342—Cross-section of mid-intestine of Acilius sulcatus, showing the arrangement of the cæca, two tracheæ passing into each cæcum.—After Plateau.
d. The excretory system (urinary or Malpighian tubes)
The excretory matters or waste products of the blood tissue of worms are carried out of the body by segmentally arranged tubes called nephridia. As a rule they arise in the blood sinuses of the body and open externally through minute openings in the skin. As there is a pair to each segment (in certain oligochete worms two or three pairs to a segment), they are often called segmental organs. In the annulate worms each segment of the body, even the cephalic or oral segment, originally contains a pair of these excretory organs. These vessels may have survived in myriopods and perhaps do exist in insects as urinary tubes, and also occur in many of the Arachnida, and thus are characteristic of each important class of land arthropods, but are either wanting or are very rudimentary or much modified in the marine classes, notably the Crustacea and Merostomata (Limulus), where they are represented by the shell-glands of Copepoda, green glands of the lobster, and the brick-red glands of Limulus.
Fig. 343.—Digestive canal of Perla maxima: l, upper lip; mh, buccal cavity; ap, common end of salivary ducts (ag); o, œsophagus; s, s, salivary glands, arranged segmentally; b, cæca of chyle-stomach; lg, their ligaments of attachment; mp, urinary tubes; r, rectum; af, anal orifice.—After Imhof, from Sharp.