SPECIES OF BRITISH II YDROPH I LUS. "5 



head and thorax are curiously marked with a number of 

 lines and spots. The legs are six in number ; they are 

 thickly set with rows of hair on their opposite sides, and 

 each is furnished with a sharp claw. The number of 

 swimmers on each side is seven ; they are covered with 

 hairs, and, to the specimen from which the drawing was 

 taken, a vast number of Vorticellce convallarice,* or bell 

 polypi, were attached. These will be recognized in the 

 magnified drawing by their bell-shaped figure. They 

 sometimes infest this species of larva to such a degree, as 

 considerably to impede its motions in swimming. On 

 each side of the abdomen, which commences near the 

 origin of the first pair of swimmers, arise the great 

 tracheae, or air-vessels, distinguishable in the coloured 

 engraving by their light blue colour ; the two approach 

 each other near the tail, where an exceedingly curious 

 process is also distinctly exhibited. The whole surface of 

 the body is thickly covered with hairs, and several tufts 

 are disposed in clusters, with some regularity, down the 

 back and sides. The flexible pulsatory organ or dorsal 

 vessel, situated at the lower part of the body, is in perpe- 

 tual motion. Its form somewhat resembles the letter S, 

 inverted : it however varies a little during its vibrating 

 motions. The use of the curious appendages at the lower 

 extremity of the body is unknown. Its tail is biforked 

 and crustaceous, and is marked as shewn in the plate. 

 In figure 31 is given a representation of the larva, of its 



* Described and figured in " Natural History of Animalcules," p. 149. 



