iv VEEMESFOEM OF BODY AND ORGANISATION 187 



and proboscidal aperture are united into a single external aperture 

 only in Amphiporus, Malacobdella, and Geonemertes palaensis. On each 

 side of the head there is often a strongly ciliated longitudinal furrow 

 or lateral cleft. The anus is terminal. In the parasitic Malacobdella 

 the posterior end forms, in front of and under the anus, a ventral 

 sucker disc. 



The body of the Nemathehma, which is covered by a rough, 

 frequently ringed cuticle, is elongated and spindle-shaped, without 

 outer appendages, and with at the most small papillae or small fringes 

 at the anterior and posterior ends. Mouth and anus are, when present, 

 terminal. The Acanthocephala possess at the anterior end a proboscis 

 which can be withdrawn into a special sheath, and which is provided 

 with hooks directed backwards ; this serves for attachment to the 

 intestinal wall of the host. The males have at the posterior end a 

 protrusible copulatory organ, which, when protruded, is bell-shaped. 



The various classes of the Annulata must be dealt with separately. 



The body of the Hirudinea is long, and generally flattened dorso- 

 ventrally ; less frequently it is cylindrical (Pontobdella, Piscicola). 

 Round the mouth there is a small oral sucker and posteriorly, always 

 under the anus, a ventral sucker, which is usually larger than the oral. 

 The body is divided by furrows into numerous consecutive rings ; 

 these do not, however, correspond in number with the inner segments. 

 In all the Hirudinea the number of the latter corresponds with the 

 number of the ganglia, and is 33. The number of rings to a segment 

 in the central part of the body is typical for the various genera and 

 species. Among the Rhynchobdellidce it is 3 (Branchellion, Clepsine), or 

 6 (Calliobdella, Ichthyobdella, Pontobdella), or 12 (Piscicola); among the 

 Gnathobdellidce it is 5. In all Hirudinea the segments are shorter at 

 both the anterior and posterior ends of the body, the number of rings 

 in them being gradually reduced. Where the separate rings of the 

 typical central segments are in any special way externally marked, 

 the marks are repeated in regular succession throughout the body, i.e. 

 when such marked rings have not disappeared in the shortening of 

 the segments. These marks take the form of tactile papillae, warts, 

 protuberances, pigment spots, nephridial apertures, etc. The large 

 protuberances on the integument of Pontobdella play a most important 

 part in the dermal respiration. In Brancliellion, on each side of each 

 ring in the middle region of the body, there is an integumental 

 appendage which functions as a gill. The appendages on the first 

 ring of each segment swell at their bases and form contractile vesicles. 



In the Chcetopoda the outer segmentation generally answers to the 

 inner. The former is most distinctly to be recognised by the arrange- 

 ment of the setae, which are repeated in strict accordance with the 

 segments. The setae generally stand in groups. Every segment, 

 except the oral, carries typically on each side two bundles of setae, one 

 dorsal and the other ventral. 



