644 



ZOOLOGY 



SECT. 



storri-, 



blastoderm of the ventral plate become divisible into three layers 

 ectoderm, endoderm, and mesoderm. The mesoderm undergoes 

 division into a series of masses which are hollowed out to form the 

 primitive segments (B) and their cavities. Embryonic membranes 

 serosa and amnion are formed as in the Insects. When about ten 

 segments have become distinguishable, the rudiments of appendages 

 (Fig. 548, C, and Fig. 549) appear in the form of hollow processes 

 of the segments on either side of the middle line. Behind the 

 rudiments of the thoracic limbs appear a series of seven pairs of 



abdominal appendages (ab ') ; 

 the first early disappear ; the 

 place of the second is after- 

 wards taken by the operculum ; 

 the third develops into the pectines. 

 The four posterior pairs become 

 aborted ; the book-lungs appear 

 as pits at their bases. 



2. DISTINCTIVE CHARACTERS AND 

 CLASSIFICATION. 



The Arachnida are air-breathing 

 Arthropoda in which the body is 

 usually distinguishable into two 

 regions cephalothorax and abdo- 

 men. The cephalothorax bears 

 sessile, usually simple eyes, two 

 pairs of jointed appendages the 

 chelicerse and pedipalpi and four 

 pairs of legs. There are no an- 

 tennae. The organs of respiration, 

 when present, are usually either 

 tracheae or book-lungs, but in the 

 Xiphosura take the form of book- 

 gills. Heart and vascular system 

 are usually present ; the heart is 

 tubular, like that of the Insects. 

 The sexes are nearly always separate, and there is usually no meta- 

 morphosis. 



The class is divided into the following orders : 



ORDER 1. SCORPIONIDA. 



Arachnida in which the body consists of a continuous cephalo- 

 thorax and an abdomen, the latter consisting of an anterior broader 

 pre-abdomen of seven segments, and a posterior, narrower post- 

 abdomen of five segments, with a caudal spine in the form of a 

 sting. There are small ch elate chelicene and large chelate pedipalpi. 



FIG. 549. Embryo of Scorpion (Euscor- 

 pio carpathicus), with rudimentary 

 appendages, ab 1 - 7 , abdominal appen- 

 dages ; ch, chelicera ; ch.g, rudiment of 

 cheliceran ganglion ; c.gr, cephalic 

 groove ; c.l, caudal lobe ; A- 4 , walking 

 legs ; fed. pedipalp ; stom, opening of 

 stomodsRUin ; v.tf, ganglia of ventral 

 nerve cord. (From MacBride, after 

 Brauer.) 



