In the peilect Insect the varieties of form are still more numerous and perplexing. Besides the complicated prehensile and dental apparatus, there is often an cesoph agus, a crop, a muscular gizzartl, a stomach, a small intestine, a large intestine, and a narrower rectuni. But development is manifested, not only by differ ences in the diameter and structure of dif ferent lengths of the tube itself, but also by its complication, through the addition of supple mentary organs of a more or less tubular form.
The ingluvies or crop is present in many but not all of the suctorial genera. It is sometimes distinctly glandular. And even where, as in the Bee, this character is less prominent, it is still probably a secreting organ. But its uses seem to be mainly those of accu mulation.
The gizzard is generally added to the former organ. It is characterized by distinct mus cularity, and a more or less hard or horny epithelium, which is often developed into plates, protuberances, hairs, or teeth. Some times it is only rudimentary :—a toothed cesophagus subserving its functions in some insects ; while in others, it is reduced to a mere thickening of the muscular wall of this part of the canal.
The stomach is also of various form and size. Ia some insects it is simple ; in others it is more or less plicated or cellulated, or its cells are even prolonged into short cmca.
The peculiarities of the remaining subdivi Sions of the canal are chiefly those of their length and width, and in the degrees of con striction by which they are separated from each other. As yet, however, it has been found im possible to make out any intimate connection between these differences in the anatomy of the tube and the habits of the animal possess ing it. Indeed, the general relations of this kind seen in other orders often seem to he interrupted or even reversed in the insects.
The numerous tubes which open into the intestinal canal present still more diver sity. They are often named salivary, biliary, or urinary organs. Thus those tubes which open into the earlier part of the intestinal cavity are called salivary ; those which empty themselves into the commencement of the small intestine are regarded as biliary; and, finally, those which open into the canal at or near its termination, are considered urinary. It is
only the first of these that, after many grada tions, fairly attain the glandular development which a conglomerate condition implies. The second vary chiefly in number, and in the fre quency of their anastomosis. The third are rarely vesicular in shape.
The digestive canal of the Arachnida offers, on the whole, more uniformity. The chief divisions of this order are the parasitic, the spiders, and the scorpions. All are " carni vorous :"—a term which here, as often else where, is only approxiinatively correct; since most of them do not devour the flesh, but rather suck the juices, of their casual or more permanent victims.
The siniple digestive tube of the Acari or Mites is prolonged in a straight line from mouth to anus. It is sometimes complicated by gastric cwca or dilatations.
In the Aranei, or spiders proper, a slender cesophagus passes back from the mouth to a " stomach." This is sometimes a mere dila tation; sometimes is indicated by four sacculi, that radiate from a narrow tube; and sometimes presents a cavity, having blind prolong,ations that extend into the bases of each of the maxillary palpi and thoracic legs. All these parts occupy the anterior Or cephalo-thoracic division of the body.. The remainder of the canal, entering the abdominal segment, dilates, after a single convolution, into a large and sometimes globular intestine, to reach the anus by a short portion, of narrower diameter, called a rectum. The long tubes met with in the Insects recur in this order. One set, of vary ing size, open in the neighbourhood of the complicated apparatus of prehension ; these, from their position, are supposed to be sali vary. And occasionally a special poison gland appears to empty itself in this neigh bourhood. A middle set, called hepatic, often forms two pairs of tubes, with orifices much posterior to the gastric sacculi ; in other cases they are very numerous, and are con cealed by a granular mass, which occupies the same situation. The posterior set are one or two pairs of long exca, which join the intestinal cavity close to its terrnination, and are hence compared to urinary organs.