178 THE CRUSTACEA 



Fine capillary networks surround and penetrate the optic 

 ganglia and those of the thoracic region. Converging venous 

 channels on the sides of the thorax convey the blood from the 

 limbs to the pericardium. Small papilliform elevations of the 

 integument on the course of these channels in Myds, have been 

 credited with a branchial function. The chief seat of respiration 

 in the Mysidae, however, appears to be the carapace, in which is a 

 rich network of blood-channels receiving blood from the sinuses of 

 the anterior part of the body and returning it to the pericardium. 

 The epipodite of the maxilliped may also have a branchial function, 

 and at all events serves, by its movements, to maintain a current 

 of water under the wings of the carapace. The branchiae of the 

 Lophogastridae and Eucopiidae have already been described. 



Excretory System. — The antennal gland is well developed in 

 Mysis. The canal is much convoluted and expands into a small 

 bladder before opening to the exterior on the second segment of 

 the antennal peduncle. Groups of excretory cells are present also 

 at the bases of the thoracic limbs. 



Nervous System. — The oesophageal connectives are elongated 

 and a post-oesophageal (antennal) commissure appears to be present. 

 In Boreomysis the full number of eleven pairs of ganglia can be 

 distinguished in the cephalothoracic part of the ventral chain, but 

 in Mysis all are coalesced into a continuous mass within which only 

 ten pairs of ganglia can be made out. In Gnathophausia the first 

 three pairs are completely coalesced and the fourth is closely 

 approximated to them, but the remaining seven pairs are distinct. 

 In all cases six abdominal ganglia are present. 



Sense-Organs. — The eyes have the cornea slightly faceted ex- 

 ternally. The crystalline cone is bipartite and the elongated 

 rhabdome is quadripartite. In certain bathypelagic Mysidae the 

 ommatidia are divided into two groups differing in structure. In 

 Gnathophausia there is, on the upper surface of the ocular peduncle, 

 a small prominence which probably corresponds to the sensory 

 papillae found in the Leptostraca. 



The statocyst, which is found in the endopodite of the uropods 

 in nearly all Mysidae (Fig. 108), consists of a spacious vesicle 

 originating as an invagination of the integument and remaining in 

 communication with the exterior (in some species at least) by a 

 narrow fissure. It contains a single large discoidal statolith {st), 

 consisting of an organic nucleus surrounded by a thick shell of 

 calcium fluoride and resting on a group of setae springing from 

 the floor of the cavity. The tips of the setae are imbedded in 

 the substance of the statolith. Each statocyst is supplied by a 

 large nerve (?j) from the last pair of abdominal ganglia. 



Holt and Tattersall have observed in Hansenomysis a pit on the 

 upper surface of the proximal segment of the antennule. Although 



