Development of the Spiders. 211 



to the cephalic portion : further, the muscular thickening at 

 this point is such that the correspondence of the anterior 

 thoracic plate and the cerebral ganglia is entirely masked. 

 In the abdomen the peripheral musculature is represented by 

 three groups of muscles (dorsal, ventral, and lateral) — the first 

 formed at the expense of the mesoderm which surrounds the 

 dorsal vessel, the other two at that of the ventral mesodermic 

 mass. 



A last fact which deserves to be mentioned is the consider- 

 able number of ganglionic masses of the thoracic region. 

 The nervous bands, in dividing, form, not only the five pairs 

 of large ganglia which persist in the adult, but also three or 

 four smaller pairs applied to the lower surface of the peduncle 

 and which are very visible shortly after exclusion ; these are, 

 no doubt, ganglia of abdominal segments which have concen- 

 trated themselves towards the thoracic region. 



In conclusion, I may notice some interesting retrogressions 

 which take place towards the close of the passage to the adult, 

 such as the degeneration of the four diaphragmatic parti- 

 tions, of the dorsal portion of the peripheral musculature 

 of the abdomen, and of a considerable part of the nervous 

 system. 



EXPLANATION OF PLATE IX. 



Fig. 1. Limuloid stage, seen from the back. 



Fig. 2. The same, seen in profile. /, labium ; I c, cephalic lobe ; b, 

 mouth ; v o, sternal vesicle ; b v, limit of the extension of the 

 tergal plate. 



Fig. 3. The same, seen on the ventral surface, and showing the distri- 

 bution of the mesoderm, as well as the limits of the vitel- 

 line vesicle. I, labium ; o b, labrum and mouth ; b n, nervous 

 band. 



Fig. 4. Section a little below the mouth, ch, chelicerae ; ee, oesophagus ; 

 mmm, the three superficial parts of the germinative bands 

 carried on in the movement of the oesophagus ; gn, cerebral 

 ganglia. 



Fig. 5. Section at the level of the labium ; the nervous bands, bn, are 

 still adherent to the blastoderm, ch, chelicerse ; /, labium ; gn, 

 cerebral ganglia ; in, as in tig. 4 ; bn, nervous bands. 



Fig. 6. Diagram of the musculature in the thoracic region (theoretical). 

 m, central musculature ; m', peripheral musculature ; c d, diges- 

 tive tube ; gn, ventral ganglia ; ex, space in which the stomachal 

 caeca will be formed. 



