732 DR WALTER E. COLLINGE. 



Richardson, Paridotea, Stebbing, Zenobiana, Stebbing, Glyptidotea, Stebbing, 

 Cleantiella, Richardson, Engidotea, Barnard, and Pentias, Richardson, five-jointed ; 

 and in Glyptonotus, Eights, the number varies from three to five joints. 



(c) The Mesosome. 



The mesosome or thorax consists of seven free segments articulating with each 

 other. An exception to this is found in Crabyzos longicaudatus, S. Bates (2), where 

 the cephalon is partially coalescent with the first mesomatic segment. 



The first segment is usually narrower than the rest, and partly surrounds the 

 cephalon. The pleural plates may be well developed, as in /. baltica, I. neglecta, 

 and /. emarginata, or very small, as in I. linearis. 



1. Coxal Plates. — In most members of this family the coxopodites of the appen- 

 dages are developed to such an extent as to protrude beyond the pleural plates or 

 overlap or replace them on the dorsal surface. Many authors have used the term 

 " epimera" for these, but, as Calman (10) points out, that term has been used in so 

 many different senses, that it seems better to abandon it altogether. In our British 

 species the greatest development of these plates is seen in I. baltica, 1. neglecta, 

 and I. emarginata, and they are smallest in /. linearis and Zenobiana prismatica, 

 and occupy an intermediate position in the members of the genus Synisoma. In 

 /. hectica (Pallas), an Atlantic and Mediterranean species, they do not protrude 

 beyond the ventral surface of the pleural plates. 



2. Appendages. — Each segment carries a pair of appendages, and each appendage 

 is composed of seven joints. The coxopodite and basiopodite may be very small, 

 whilst in the first and second appendages the distal joints are usually shorter than 

 in the remaining ones. In many species the anterior appendages are smaller than 

 the posterior ones ; on the other hand, in Crabyzos longicaudata (S. Bate) the first 

 pair are said to be the largest. The dactylopodite is chelate and unguiform. All 

 the appendages are provided with setae, sometimes of three or four kinds, and all 

 are ambulatory in character. 



In the females the third and the three following appendages each have at their 

 base a large membranous plate folded beneath the body to form an ovigerous sac or 

 marsupial chamber. 



(d) The Metasome. 



The metasome in the Idoteidse has undergone considerable modification by a 

 consolidation of the terminal segments to form a strong dorsal shield. 



The number of segments varies in the different genera : thus in Glyptonotus, 

 Eights, there are four or five segments ; in Mesidotea, Richardson, Chiridotea, 

 Harger, four ; in Idotea, Fabricius, Chiriscus, Richardson, and Pentidotea, Richard- 

 son, three are present; in Cleantiella, Richardson, there are two segments; in 

 Zenobiana, Stebbing, three to five may persist; whilst in Synidotea, Harger, 



