SOME OTHER CRUSTACEA. 215 



of the genus, but it will be found that the telson ends in 

 four minute spines, separated by two setae from the large 

 lateral spines. 



The above may serve as examples of our British Mysidse, 

 and will show how relatively small are the differences which 

 separate the species and even the genera, compared with 

 the differences between the sexes. When to this is added 

 the fact that many of them only appear sporadically and 

 locally on our coasts, it will be readily understood that not 

 only have males and females been commonly referred to 

 different genera, but also that the different specialists in the 

 group have held very various views as to what should con- 

 stitute generic or specific distinctness. Consequently there 

 is great confusion as to the names of the different forms. 

 For example, a form described in Bell's Crustacea under the 

 name of TTiemisto brevispinosa appears to be only the male 

 of My sis flexuosa. 



The other Schizopods lie somewhat outside our range, for 

 they inhabit the open sea. The interest of the order as 

 a whole lies in the general resemblance to the Natant 

 Decapods, and the detailed similarity to the larvae of many 

 of the Decapods (see Fig. 59). The beauty of the form 

 and colour, the activity, the frequent extraordinary abund- 

 ance of individuals, and the habit of swimming in shoals 

 should also be noticed. 



The orders Decapoda and Schizopoda, whatever their 

 other differences, both include forms having stalked eyes 

 and a dorsal shield or shell, but there are other shore 

 Crustacea of considerable size and complexity in which the 

 eyes are sessile and the dorsal shield is absent. These fall 

 into two sets: (1) the Isopoda, forms more or less like the 

 common "slater," or wood-louse, with flattened bodies, and 

 (2) the Amphipoda, or sand-hoppers, whose bodies are com- 

 pressed, and who usually have six abdominal legs, three 

 directed forwards and three backwards. We shall not 

 enter into either of these orders in detail, for their mem- 

 bers are not as a rule attractive to most people, and are 

 often difficult of identification. 



As an example of the Isopoda we may take a not un- 

 common and somewhat interesting form known as Idotea 

 tricuspidata. It is usually found clinging to weed, especially 



