228 INVERTEBRATE MORPHOLOGY. 



II. CLASS Hirudinea. 



The Hirudinea differ from the Chaetopoda iu their external 

 form, being destitute either of parapodia or setae, and possess- 

 ing at the anterior end of the body a muscular sucker at the 

 bottom of which the mouth is situated, while a second larger 

 sucker used for attachment occurs at the posterior extremity 

 of the body. The outer surface of the body is distinctly 

 ringed, but a comparison of the rings with the internal organs 

 shows that they have not a inetarneric value, but that a num- 

 ber of them, varying in different forms, are included in each 

 true segment of the body. In Brancliellion and Clepsine three 

 such rings correspond to a rnetarnere, in Ichthyobdella and 

 Pontobdella six, in Piscicola twelve, and in all the group of 

 the Gnathobdellidae five. Towards the anterior and posterior 

 ends of the body a reduction of the number of rings corre- 

 sponding to a metamere is found, as for instance in the genus 

 Macrobdella (Fig. 106), which has in the middle region five rings 

 to a segment. The first two metanieres consist of but one ring 

 each, the third of two rings, the fourth, fifth, and sixth each 

 of three rings. At the posterior end of the body the twenty- 

 third metamere consists of four rings, the twenty-fourth, 

 twenty-fifth, and twenty sixth of two rings each ; while prob- 

 ably no less than seven metanieres whose rings are not 

 readily distinguishable are represented in the posterior sucker. 

 The entire animal consists, therefore, of thirty-three rnetaineres, 

 and this number is characteristic for all the Hirudinea a 

 defiuiteness of number which contrasts strongly with the wide 

 variations found in the Chaetopoda. This number does not 

 include a small lobe in front of the most anterior metamere, 

 which may be equivalent to the prostomial lobe of the Oligo- 

 chaeta, and may represent another metamere. 



As in the Oligochaeta the gland-cells of the hypodermis at 

 about the time of reproduction become enlarged and more 

 abundant in a definite region of the body, forming a clitellum 

 which is usually in the neighborhood of the tenth, eleventh, 

 and twelfth metameres. As a rule no branchiae occur, though 

 an exception is found in the marine genus Brancliellion, in 



