XVII ANATOMY 



445 



that point. Immediately afterwards it dilates into a flask-like 

 gastric sac which occupies almost the whole width of the abdomen, 

 and proceeds straight to the anus. Viewed from above, the sliape 

 of this sac is entirely concealed by the large number of caeca 

 (thirty) to which it gives rise dorsally and laterally. The two 

 largest of tliese caeca extend, parallel to each other, over the 

 whole of the abdominal portion of the gastric sac, and are flanked 

 by four lateral pairs of smaller caeca, while there is a cluster of 

 small caeca covering the anterior and narrower portion of the 

 flask-like stomach. 



The large hepatic mass so conspicuous on opening dorsally 

 the abdomen of a Spider is here entirely absent, but its functions 

 are believed to be performed by certain wrinkled, tubular, longi- 

 tudinally parallel bodies, about seven in number, closely applied 

 to the under surface of the flask. 



The masticating portions of the maxillae of the pedipalpi and 

 the first pair of legs are hollow distensible sacs, often seen in a 

 swollen condition in specimens kept in spirits. They are 

 furnished, on the inner surface, with a horny ridge. 



Owing to the fixity of the coxae of the legs, their maxillary 

 plates are incapable of much lateral motion, but are rubbed 

 against each other vertically. 



Beyond the fact that the heart is a dorsal tube lying along 

 the anterior two-thirds of the alimentary canal, and divided by 

 constrictions into three well-marked and equal portions, little is 

 known of the l)lood - system of these animals. It is probably 

 essentially like that of Spiders, but the presence of a pericardial 

 sac has not yet been established, nor has the course of the blood- 

 vessels been described in detail. 



As in other Arachnids, the principal ganglionic nerve masses 

 closely embrace the oesophagus. Immediately anterior to it, 

 forming a conical mass with its base on the oesophagus, is the 

 cerebral ganglion, while just behind it is the transverse portion 

 of the large thoracic nerve-centre. In Phalangium opilio, accord- 

 ing to Tulk,^ a median nerve is given off from the apex of the 

 cerebral mass (the paired nature of which is apparent) and bifur- 

 cates to the two eyes. Two lateral nerves proceed to certain 

 organs near the origin of the second pair of legs, which were 

 tbought by the old writers to be lateral eyes, but wliich are now 

 ' Mag. Nat. Hist, (i.), xii., 1843, p. 325. 



