A REVIEW OF THE MYSIDACEA 11 



SYSTEMATIC DISCUSSION 



Class CRUSTACEA 



Subclass MALACOSTRACA 



Division Peracarida 



Carapace, when present, leaving at least four of the thoracic somites 

 distinct; first thoracic somite always fused with the head; proto- 

 podite of the antenna typically of three segments; mandible with 

 lacinia mobilis (except in parasitic and other modified forms), and a 

 spine row between the incisor and molar parts; maxillule with lobes 

 usually from the first and always from the third joints; maxilla 

 usually with exopod attached to the third joint; thoracic limbs flexed 

 between the merus and carpus as in the Syncarida and Eucarida; 

 oostegites attached to some or all of the coxal joints of the thoracic 

 limbs in the female, forming a brood pouch; no appendix interna on 

 the pleopods; hepatic caeca few and simple; heart generally elon- 

 gated, extending through the greater part of the thoracic region, or 

 displaced into the abdomen ; spermatozoa generally filiform ; develop- 

 ment taking place in the brood pouch ; young set free at a late stage. 



Hansen (1925) has demonstrated quite conclusively that the flexure 

 in the endopod of the thoracic limbs (knee) in the Peracarida occurs 

 between the merus and carpus as in the Syncarida and Eucarida. 

 It was formerly supposed that the knee joint occurred between the 

 carpus and propodus, and that this constituted a character in which 

 the Peracarida differed from the other two divisions (Caiman, 1904). 

 Hansen is also of the opinion that in the endopod of the thoracic limbs 

 of the Peracarida there is a joint, called by him the prae-ischium, 

 interpolated between the basis and ischium. 



A further point of great morphological interest is raised in Hansen's 

 survey of the thoracic limbs of the Crustacea, namely, the demonstra- 

 tion of a procoxal joint in some of the thoracic limbs of some of the 

 Peracarida. This discovery means that in the thoracic append- 

 ages, at any rate, there is a 3-jointed protopoclite, composed of 

 precoxa, coxa, and basis. Hansen did not attempt to extend this 

 interpretation of the protopodite to the head appendages but it 

 seems to me that if this were done it is possible to unify the 

 morphological conception of all the appendages to a common 

 scheme, based on a 3-jointed protopodite. For instance, Hansen 

 states that in the maxilla of the Peracarida the exopod is attached 

 to the third joint, to which also the endopod is attached. If we 



