Читать книгу Clinical Guide to Fish Medicine - Группа авторов - Страница 45

The gastrointestinal (GI) tract is short and simple, essentially a slightly S‐ or J‐shaped tube. The ileum includes a spiral or valvular intestine that significantly increases surface area for absorption. This structure is colloquially referred to as a spiral colon but is not technically colonic tissue. The valvular intestine has four distinct variations: a spiral winding around a central column, cones directed caudally or cranially, and scroll‐shaped (Hamlett 1999). The valvular intestine empties into a short tube interchangeably termed the colon and rectum (Wood et al. 2007; Theodosiou and Simeone 2012). A few elasmobranch species have a pyloric cecum or ceca, e.g. deepwater dogfish (Somniosus spp.) (Hamlett 1999).

The rectal gland is a unique intestinal appendage located caudal to the valvular intestine at the colon/rectum (Figure A9.7). It has an osmoregulatory function and the fluid secreted contains sodium and chloride at concentrations almost twice those of the plasma; this is in contrast to elasmobranch urine which is not concentrated (Shuttleworth 2012). The rectal gland is significantly reduced in freshwater elasmobranchs (Evans et al. 2004).

There are bilateral coelomic (abdominal) pores in the cloaca suspected to have an excretory function (Figure A6.6). They provide a possible access point for catheterization or endoscopy of the coelom.