220 > INVERTEBRA TE MORPHOL OGT. 



A circulatory system is always present, and consists of a 

 dorsal longitudinal vessel lying on the dorsal surface of tlie 

 digestive tract and a ventral one lying below it, the two 

 being united in more or fewer of the metameres hj one or 

 two lateral vessels on each side. The dorsal vessel is con- 

 tractile, the blood in it flowing towards the anterior extremity, 

 and the lateral vessels are also usually contractile. Branches 

 are given off to the muscles and to the various organs in reg- 

 ular metameric succession, and additional longitudinal vessels 

 are also found accompanying the ventral nerve-cord, two 

 lateral and one ventral. In the terrestrial forms fine branches 

 penetrate into the hypodermis, the aeration of the blood being 

 thus effected in the absence of special branchiae. The blood 

 in the majority of forms is red from the presence of haemo- 

 globin dissolved in the plasma and contains colorless cor- 

 puscles. As in the Polychaeta the ccelom contains a hsemo- 

 lymph in which corpuscles float. 



The digestive tract forms a straight tube, extending from 

 the mouth, situated on the ventral surface at the junction of 

 the prostomium and first trunk metamere, to the terminal 

 anus. The mouth opens into a short mouth-cavity, and this 

 into a more or less muscular pharynx, which in most cases 

 can be protruded from the mouth and is slung to the body- 

 wall by numerous radiating muscular bands. To it succeeds 

 a smaller oesophagus, which communicates posteriorly in ter- 

 restrial forms, after in some cases dilating to form a sack-like 

 thin-walled crop, with a muscular gizzard. The intestine 

 which succeeds this is usually somewhat pouched, being con- 

 stricted in the region of the dissepiments and bulging out 

 into the intervening coelomic cavities. In the terrestrial Oli- 

 gochaets its absorptive surface is increased by the projection 

 into it along the dorsal surface of a longitudinal fold, the 

 typMosole, the chloragogue cells lying in the furrow produced 

 by the fold. Various glands open into the digestive tract at 

 different regions, as, for instance, salivary glands which open 

 into the anterior part of the oesophagus in some forms, and 

 calciferous glands (Morren's glands) which contain particles 

 of carbonate of lime and are found opening into the oesopha- 

 gus in terrestrial forms. 



