eventually with the finer ductules, which, by their union, form the main efferent duct. THE PYLORIC CAECA.—These structures are caecal outgrowths from the intestine, and are situated close to the pyloric extremity of the stomach and the intestinal apertures of the bile and pancreatic ducts. Wholly wanting in the Cyclostomata and Dipnoi, and, unless represented by a pair of caeca opening into the long, tubular, non-valvate anterior portion of the intestine in the Greenland Shark (_Laemargus borealis_),[267] in the Elasmobranchs also, they are very generally present in the Teleostomi, although extremely variable both in number and arrangement in different families. In _Amia_ there is no trace of pyloric caeca. _Polypterus_ has a single short caecum with a thick muscular wall. In _Acipenser_, _Polyodon_, and _Lepidosteus_, on the contrary, pyloric caeca are unusually well developed. In _Acipenser_ the caeca are not only numerous, but are so connected together by connective tissue and blood-vessels, and so invested externally by the peritoneum, as to form a large, compact, gland-like mass, communicating with the intestine by a single wide duct. In _Polyodon_ the organ is essentially similar, but is lobed externally. In _Lepidosteus_ (Fig. 155, B, _py.c_), the caeca are also very numerous, but relatively short, and, although united into a compact mass, open by four pit-like orifices into the intestinal cavity. In Teleosts the caeca are subject to extraordinary variations in number, size, and arrangement.[268] In some families, and even in groups of higher taxonomic value, they are entirely absent, as is the case with the {275}Siluridae, Esocidae, Cyprinodontidae, Labridae, Plectognathi, and Lophobranchii. The "Sand-eel" (_Ammodytes_) has but a single caecum; the Turbot (_Rhombus maximus_) two, and other Pleuronectidae three to five; and the Perch (_Perca_), three (Fig. 160, _py.c_). In other Teleosts, on the contrary, these structures are much more numerous. In _Labrus labrax_ there are about 60, in the Whiting (_Gadus merlangus_) 120, while in the Mackerel (_Scomber scombrus_) there are no fewer than 191. If few in number the caeca open separately into the intestine, but when numerous, more or fewer of them may unite to form a smaller number of efferent ducts, as in the Whiting, where four such ducts are formed. In some instances, as in the Tunny (_Thunnus_), the union of the caeca by connective tissue leads to the formation of a compact mass. As regards their arrangement, the caeca may either be disposed in a whorl round the intestine, as in the Whiting, or in a linear series, as in the Salmon (_Salmo_) and in some of the Clupeidae. The mucous membrane lining the anterior pyloric caeca is often developed into a network of ridges, limiting crypt-like or tubular depressions; and not infrequently the epithelium is ciliated.