240 ZOOLOGY FOR MEDICAL STUDENTS chap. 



Peripatus (Fig. 95, p. 211) still retains the elongated worm-like form 

 of body with simple stump-like appendages similar in character through- 

 out the length of the body, the soft muscular body-wall without marked . 

 thickening of the cuticle^ and the nephridia distributed in pairs segmentally 

 throughout the length of the body. On the other hand it has developed 

 the typical arthropodan characteristics that a pair of appendages have 

 become converted into jaws and that the coelomic body-cavity has 

 shrunk up and been replaced by a haemocoelic body-cavity filled with 

 blood and in free communication with the interior of the heart by means 

 of ostia. It possesses also the arthropodan character that it breathes by 

 tracheal tubes. Further, although the number of segments in the body 

 as indicated by the appendages differs in different species and even, to 

 a less extent, in different individuals of the same species, this number 

 remains fixed throughout the life of the individual from an early stage 

 of embryonic development. 



Peripatus is then on the balance of its structural features to be classed 

 as an Arthropod. That it is a very primitive arthropod is shown — apart 

 from the annelidan features already mentioned — by the relatively feeble 

 elaboration of the head region. Only a single pair of appendages have 

 become modified as jaws ; there is none of that crowding together of 

 appendages in the neighbourhood of the mouth and their modification 

 for different functions that is so striking in the typical arthropod. 

 Nor is there the correlated crowding together of the corresponding nerve- 

 ganglia to form a complex brain. 



Over fifty species of Peripatus are known, distributed in seven 

 different centres in the warmer parts of the world (Mexico and West 

 Indies to Rio ; Congo ; Malaya ; South Africa ; New Britain ; Austral- 

 asia ; Chile). Such wide discontinuous geographical distribution is a 

 feature commonly met with in ancient types of animal. 



Peripatus lives in damp localities and during the day is to be found 

 lurking under logs, bark or stones. It may be recognized by its general 

 shape, its velvety skin, and its habit of ejecting sticky slime from the 

 ends of the appendages next behind the jaws. 



II. Under the name Myriapoda are included a number of groups of 

 terrestrial arthropods which still retain the elongated form of body 

 without differentiation into distinct regions but which in other respects 

 are highly developed arthropods. They are exemplified by the carnivor- 

 ous Centipedes and the vegetarian Millipedes. The former possess poison 

 glands opening at the tip of the fourth pair of mouth appendages which 

 have the form of claw-like poison-fangs. In the case of the large tropical 

 centipedes the bite may be dangerous to man. 



