438 INVERTEBRATE MORPHOLOGY. 



embryonic life being, however, distinct and postoral in posi- 

 tion, only later moving forward. The succeeding ganglia are 

 generally more or less fused, and indeed in some forms all 

 the ganglia of the limb-bearing segments of the cephalothorax 

 may be united with those of the abdominal region to form a 

 single gangliouic mass. In some forms a single ganglion 

 occurs behind this mass at the junction of the cephalothorax 

 and the abdomen, and in the Scorpions there is posteriorly a 

 ventral nerve-cord with seven pairs of ganglia, the anterior 

 pair corresponding with the fifth abdominal segment. A 

 sympathetic nervous system occurs in the Scorpions, Spiders, 

 and Harvest-spiders, consisting of a nerve arising from the 

 syncerebrum and passing to the digestive tract. 



Hairs situated upon the body and appendages serve as 

 sense-organs of touch and apparently also of audition, since 

 Spiders are sensitive to air-vibrations and possess no definite 

 auditory organs. Eyes are very generally present and vary 

 considerably in number, there being in the Spiders three or 

 four pairs ; in the Scorpions, in which there are from two to 

 six pairs, one pair become closely approximated on each side 

 of the mid-line and recall the median eyes of Limulus, while 

 the remaining pairs are situated more to the sides of the 

 cephalothorax. In structure the median eyes differ from the 

 lateral ones ; the chitiuous cuticle is thickened over them to 

 form a simple unfaceted lens below which lies a layer of 

 transparent cells continuous with the general ectoderm (hypo- 

 dermis) of the body and which may be termed the cornea! 

 hypoderinis, though more usually they are known as the 

 vitreous cells. Below them comes the retina, consisting of a 

 single layer of elongated cells with their nuclei situated 

 toward their inner ends, with which the fibres of the optic 

 nerve come into connection. The various retinal cells are 

 arranged in groups of five (retinulse), which secrete a thin 

 chitinous rod upon their contigiious faces, producing thus a 

 rhabdom composed of five parts. Behind the retina is a thin 

 layer of cells, the postretiuular layer, and numerous pigment- 

 cells occur between the various retiuulse. The lateral eyes 

 are constructed upon a very different plan, lacking a corneal 

 hypodermis between the retina and the cornea. They are 



