THE CCELOMATA INVERTEBRATA 173 



similar position on the segment in front are two small holes, the 

 openings of the female ducts, the oviducal pores. Lastly, there are 

 two pairs of minute openings, the spermathecal pores, lying in the 

 Central part of the grooves between segments 9 and 10 and 10 and n, 

 slightly to each side of the middle line. They open into the sper- 

 mathecae, whose function we shall deal with later. 



Two of the main features that will at once be seen on 

 opening up an earthworm have already been alluded to, namely, 

 the fact that the alimentary canal is a tube lying in the coelom, and 

 that the coelom itself is cut up by a number of septa into separate 

 compartments. The thin muscular septa run from the intersomitic 

 groove to the gut wall, into which they are inserted, causing as a rule 

 a constriction. They are perforated by small holes, so that the 

 various compartments of the coelom are not absolutely isolated from 

 one another. Septa are absent in at any rate the first two or three 

 somites. 



The alimentary canal is a straight tube running from mouth to 

 anus and over the greater part of its course is alike, but at its anterior 

 end it is much modified. As we have seen, the mouth is situated 

 on the ventral side of the first somite, and it leads into a wide thin- 

 walled receptacle, the buccal cavity, filling the first two somites. 

 Following this is a large thick- walled, muscular portion of the canal, 

 the pharynx, which extends back into the sixth or seventh segment, 

 but is really in front of the septum between 5 and 6. From it 

 radiate out muscular strands, some even piercing the septa behind, 

 by means of which its cavity can be enlarged, and in this way it 

 can act as a suction pump, drawing in anything to which the mouth 

 has been attached. Buccal cavity and pharynx together constitute 

 the first part of the alimentary canal, the fore gut or stomodceum, 

 and are marked off from the next portion by the fact that they are 

 formed by an ingrowth of ectoderm, whereas the succeeding parts 

 of the gut are derived from the entoderm, an important distinction. 

 The oesophagus is a straight narrow tube running back from the 

 end of the pharynx to the fourteenth somite. In the eleventh and 

 twelfth segments three small sac-like swellings, the cesophageal glands, 

 are to be found on the sides of the oesophagus. The first pair are 

 actually pouches, cesophageal pouches, opening directly into the 

 oesophagus, and the last two pairs are thickenings of its walls, only 

 communicating with it via the pouches, \\ithin, they are divided 

 by partitions Into small chambers, and they produce a whitish fluid 

 containing a multitude of small calcareous particles excreted by the 

 large cells which line them, and hence their name calcareous glands. 

 In segments 15 and 16 the alimentary canal is dilated to form 

 a large thin-walled distensible sac, the crop or proventriculus. 



