Page:Popular Science Monthly Volume 9.djvu/738

710 Among Arachnida forms of the nervous system exist which agree in many respects with those belonging to members of the class last described—these resemblances being in the main associated with certain general similarities of external form or configuration of body. Thus in scorpions the arrangement of the nervous system is not very dissimilar from that belonging to the prawn and its allies, since the thoracic ganglia have coalesced with one another and with the anterior abdominal ganglia, so as to form a large stellate nervous mass, which supplies the limbs and the anterior part of the abdomen. The ventral cord throughout the remainder of the abdomen and its caudal prolongation is marked at intervals by a series of small ganglionic swellings.

In spiders proper the nervous system attains its maximum of concentration. In addition to the abdominal and thoracic ganglia having all fused into one another and with the sub-œsophageal ganglion, we

find the large mass thus composed (Fig. 5, s) brought into extremely close relation with the cerebral ganglia or brain (c). They are connected by means of two stout commissures, one on each side of the very narrow œsophagus, whose small size is attributable to the suctorial habits of these carnivorous and predatory creatures. The captured fly is not eaten, its juices are sucked by the fierce spider by whom its life has been taken.

The bilobed brain of the spider receives nerves on each side (o), corresponding in number with the ocelli which the animal may possess. It also receives two large nerves (m) from the so-called mandibles, which are organs presumably developed from modified antennas. These large nerves probably contain outgoing as well as ingoing fibres.