202 DESCRIPTIONS OF PREPARATIONS. 



somites the nephridia or excretory organs are left in situ on either side of 

 the intestine. 



The stomodaeum in Aeolosoma is restricted to the first somite of the body ; 

 in other Oligochaeta it extends backwards through a variable number of somites, and 

 is differentiated into a buccal cavity and a musculo-glandular pharynx. The muscular 

 tissue is most developed on the dorsal aspect of the pharynx, and is irregular in 

 arrangement. The pharyngeal glands appear to correspond to the septal glands of 

 the Enchytraeidae : they are large in Megascolex Moseleyi and Pontodrilus. 



The oesophagus and stomach-intestine jointly represent the archenteron. The 

 oesophagus of Perichaeta has three sets of glands attached to it. The gizzard is 

 absent in Pontodrilus and all aquatic Oligochaeta except some Naidomorpha. It is 

 in some instances placed more anteriorly in the body, e. g. in Urochaeta, Antaeus, 

 &c., than it is in Lumbricus. Digaster possesses two, Moniligaster five, gizzards. 

 The second gizzard in the former is separated by a whole somite from the first : 

 the last four gizzards of Moniligaster follow one another closely, but are six somites 

 behind the first. In these cases a longer or shorter tube intervenes between the 

 gizzards, as is the case also between the gizzard and the sacculated region of the 

 intestine. It is difficult to say whether the last-named tubular region should be 

 considered as intestine or oesophagus. Glands are attached to it, e. g. three pairs 

 in Urochaeta, one pair in Microchaeta, which appear to bear a lime-secreting function. 

 They may be regarded as either homologues or analogues of the glands of Morren, 

 according to the view which is taken as to the oesophageal or intestinal nature of 

 the tube from which they originate. Calcigerous glands are entirely absent in some 

 instances, e.g. in Megascolex, Typhaeus, Pontodrilus, &c. A pair of ventrally placed 

 caeca open into the commencement of the sacculated intestine of Perichaeta, whilst 

 Typhaeus and Megascolex possess a series of dorsal paired glands, five in the former, 

 fifteen to sixteen in the latter, opening into it more posteriorly. 



The walls of the digestive tract consist of the following layers from without 

 inwards : (i) an epithelial peritoneum ; (2) a longitudinal, (3) a circular, layer of 

 muscle-cells ; (4) a layer of capillary bloodvessels, and (5) the epithelium. The 

 peritoneal cells in the region of the stomach and the stomachal vessels are pyriform 

 and branched at their attached ends. They are coloured in all instances green, 

 black, or in the Earthworm brownish-yellow : hence chloragogen cells. They have 

 usually a well-defined membrane, and contain concretions. They appear to be set 

 free into the coelome, and then degenerate ; and their products, soluble and in- 

 soluble, are probably excreted through the nephridia. The longitudinal layer of 

 muscle-cells is thin, but is better developed in the gizzard where the circular 

 layer attains an extraordinary development. The capillaries are well developed 

 everywhere, but more especially in the pharynx and oesophagus. The lining epi- 

 thelium is columnar. In the Earthworm it secretes a cuticle in the pharynx, 

 oesophagus, crop and gizzard, but in the stomach is ciliated. In many of the lower 

 Oligochaeta it is ciliated throughout the whole tract. The cells in the oesophagus 

 from the i I th to the 13*^ somite are glandular. To this region belong the calci- 

 gerous glands. The first pair of these glands in the Earthworm is a pair of hollow 

 diverticula, whilst the second and third pair consist solely of enlarged gland-cells 

 (Claparede). They secrete a milky fluid which owes its milkiness to the presence 



