METAZOA VERMES. 309 



There is a marine leech, Pontobdella muricata Linn. 

 (No. 766, No. 767, model) which not only attaches itself 

 to another animal, the skate, .but even fastens its eggs to 

 the fish, showing that the parasitic habit has become 

 stronger in this leech than in Hirudo which lays its eggs 

 in earth, or in Clepsine which deposits its eggs in water. 



The indistinct segments of the body of the adult Pon- 

 tobdella are covered with warts, but a short distance from 

 the head a few segments are comparatively smooth. 

 There is a large anterior sucker, provided with papillae 

 round its edge by means of which the animal becomes 

 attached. 



The Hirudinia have much in common with the Oli- 

 gochaeta, as already stated, but their semiparasitic habits 

 and consequent change of environment have brought 

 about peculiar modifications of structure. Having given 

 up very largely the swimming mode of locomotion, the 

 parapodia have disappeared and some of the segments of 

 the body have become modified into an anterior and a 

 posterior sucker which are used chiefly for the purposes 

 of attachment, though they are also efficient organs in 

 creeping. 



, GEPH\REA. 



The trochophore of the Gephyrean Echiurus (PI. 768, 

 fig. i), has all the typical characters of the trochophore 

 of the Chaetopods, and it is also similar to the trocho- 

 phore of Mollusca. The larva (fig. 2) is segmented in the 

 posterior part of the body, but early in the development 

 the segments disappear (fig. 3, side view of an older stage 

 than fig. 2) and no trace of them is seen in the young 

 Echiurus (fig. 4) which is essentially like the adult. 



In this genus the mouth is at the anterior end and the 

 anus is terminal as in the Chaetopods. There are no 

 parapodia, but ventral setae occur, and two circles of 

 bristles at the posterior extremity. Besides these organs 



