PHYSIOLOGY OF THE INVERTEBRATA. 37 



worm. Nevertheless, nothing of the kind is discernible, 

 unless the cavity of the saccular embryo is an enteroccele. 

 And if this cavity is a blastocoele, and not an enteroccele, it 

 may become a question whether the tape-worms are anything 

 but gigantic morulEe, so to speak, which have never passed 

 through the gastrula stage." 



The Annelida. 



The second class of the Annuloid Series contains the follow- 

 ing orders: tlae Myzostomata, Gephyrca, Hirudinea, Oligocliata, 

 and Polychceta. 



(i) The Myzostomata are parasitic unsegmented worms. 

 There is a mouth, through which a proboscis is protruded. 

 The mouth passes into a straight alimentary canal terminating 

 in a cloaca. The alimentary canal has numerous lateral 

 caeca. 



(2) The GepTiyrea are marine unsegmented worms * with a 

 more or less cylindrical body. The oral aperture or mouth is 

 either terminal, or has a ventral aspect. In some forms the 

 mouth is provided with a proboscis, or is surrounded by 

 tentacula ; and it passes into a pharynx, which opens into 

 either a straight or coiled intestine. The anus is always 

 situated dorsally, and in Phoronis it is close to the mouth. 



(3) The Hirudinea. — The leeches are more or less segmented 

 worms, provided with a sucker at the anterior end of the 

 body. Most species have a second unperforated sucker 

 situated posteriorly, and there are a few of the Hirudinea 

 with lateral suckers. The mouth of Hirvdo medicincdis is 

 armed with chitinous teeth, and opens into the pharynx, 

 which is provided with glands. The sucking action of the 

 animal is produced by the contraction of the muscles which 

 suspend the pharynx. The pharynx passes into a slender 

 oesophagus, which leads into a very long stomach ; and from 



* In the larvEe of Chcetifera (belonging to the Oephyrea) there are traces 

 of segmentation. 



