18 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvii. 



with ectoderm; (2) a stomach also lined with ectoderm and provided 

 with a "g-astric mill," which is a chitinous apparatus consisting of nine 

 plates, seven of which lie in the anterior part, two in the floor of the 

 stomach; (3) an elongate intestine, more or less dilated at its anterior 

 extremity, and lined for the most part with ectoderm, the endodermal 

 portion extending but a short distance from its anterior connection 

 with the liver lobes. There is no c^cal enlargement at its posterior 

 extremity. The liver lobes or hepatic cseca consist of four elongate, 

 more or less coiled, backwardly directed tubes, opening into the ali- 

 mentary canal at the union of the stomach and the intestine. 



The heart is an elongate tube, varying in length in the different 

 groups, and situated in the pleon for the greater part, except with the 

 Chelifera, where it occupies a position in the anterior part of the thorax. 

 In the Asellidse the heart is confined to the thorax, extending from 

 the pleon forward. Surrounding the heart is a pericardium of con- 

 nective tissue, a cavity or chamber intervening, in which the blood 

 freely circulates. One, two, or three pairs of ostia place the heart 

 into communication with the pericardial cavity. From the anterior 

 extremity the dorsal aorta arises, sometimes being constricted off' from 

 the heart, and then again being simply an extension of the heart for- 

 ward, with no differentiation of the two parts, the one vessel gradually 

 becoming narrower from the posterior to the anterior extremity. 

 Considerable variation exists in the manner in which the aorta subdi- 

 vides. In some cases it extends forward to the lower lip without 

 branching, arteries arising from the heart itself supplying the various 

 parts of the bod3^ It may divide verj^ near the heart, each branch 

 giving off arteries, or some little distance from the heart, or it may 

 continue as a straight tube, giving off branches in each segment. Two 

 valves open into the aorta from the heart. In position the main part 

 of the circulatory system lies dorsal to the alimentary canal. 



The nervous system consists of a brain or supra-oesophageal ganglion, 

 composed of a number of more or less fused ganglia, connected by 

 oesophageal commissures with an inf ra-cesophageal ganglion ; from the 

 infra- oesophageal ganglion there extends a double ventral nerve cord, 

 connected by double ganglia more or less fused and inclosed in a 

 single nerve sheath, one ganglionic mass being present in each seg- 

 ment of the thorax, the abdominal ganglia being more or less fused, 

 so that from the fused mass it would be impossible to tell the num- 

 ber of corresponding abdominal segments. Commissural nerves are 

 given oft' in each segment from the ventral nerve cord. 



Lying in between and parallel to the commissures connecting the 

 thoracic ganglia in the region of the thorax is a nerve which has been 

 referred to the sympathetic system. It does not pass over the gan- 

 glia in each segment, but is fused with each anteriorly and starts again 

 on the other side. At its union with the ganglia fibers extend to the 

 commissures on either side. 



