2. The second structural modification of the large intestine, associated in functional significance with the caecal apparatus, depends upon the increase in the length of the proximal segment of the colon beyond the ileo-colic junction and the twisting or coiling of this segment in a more or less complicated definite manner, usually in the form of a spiral, the individual turns of the coil being held in place by the peritoneal connections. The proximal colon thus modified is admirably adapted to retard the movement of contents not yet completely digested and to increase the absorbing surface of the intestine, and hence is functionally allied to the caecal apparatus. This colic modification is found in its highest degree of development in the ruminant Ungulates, whose caecal pouch proper is also enormously developed. In these animals the colon immediately beyond the ileo-caecal junction is arranged in the form of a double spiral, the afferent (caecal) and efferent (colic) tubes alternating, and continuous with each other in the center of the coil (Fig. 465, _IV_, 5). Examples of this type of spiral colon are shown in Fig. 373 (_Bos indicus_), Fig. 374 (_Cervus sika_), Fig. 375 (_Ovis aries_), Fig. 376 (_Oryx leucoryx_). Ontogenetically the complicated spiral colon of the ruminants starts as a simple loop of the proximal colon, which, with the further rapid growth of this segment of the intestine, is bent to produce the turns of the coil as shown in the schematic Figs. 480-482. Phylogenetically the same gradual development can be traced in the vertebrate series. Perhaps the earliest tendency to structurally modify the intestine in the direction named is found in the manner in which the intestinal coils are bound together by the subperitoneal arachnoid in many Ophidians (Fig. 331). Further in the Manidae among the Edentates there is no caecal pouch, but the intestine at the ileo-colic junction is twisted into a figure 8 and held in this position by the peritoneal connections (Figs. 362 and 465, _IV_, 4). In certain Marsupials with well-developed caecal pouches, such as _Phascolarctos_ and the Vulpine Phalangers (Figs. 351 and 352), the colon immediately beyond the ileo-colic entrance is sacculated and bent in the form of a short loop. In the tapir (Fig. 377), the proximal segment of the colon forms a simple loop, whose afferent and efferent limbs are closely bound together. The arrangement of the large intestine in this animal illustrates the early embryonal stage in the development of the complete ruminant spiral coil (cf. Fig. 480). [Illustration: FIGS. 480-482.--Schematic representation of three stages in the development of the ungulate spiral colon.]