III. GENERAL REPRESENTATION OF THE FAMILY. 



In order to facilitate the use of this large section it is divided into three chapters, the 

 first of which contains a general view of the structure and development of the animals, 

 the second of their habitation, biology and distribution, the third some general observations 

 about the classification. Each of these chapters contains several sub-divisions. 



A. Structure aucl Development. 



a. The Female. 



This sex is known in all species. The body is nearly always a little flattened — 

 seldom more than a little; if seen from below or from above it is ovate or globular. The 

 young specimens are generally much longer than they are broad; the adults are now a little 

 longer than they are broad, now the reverse; sometimes their broadest dimension is a little 

 in front of, sometimes a little behind the middle. Specimens which are going to lay, or 

 have commenced laying eggs, are always somewhat — .probably as a rule much — larger 

 than old ones which are emptied of eggs (pi. 6, fig. 3 a shows such a female which is going 

 to lay eggs, fig. 3c a female (with a male) which is emptied of eggs, both enlarged to 

 the same scale). In consequence of this evacuation the animals frequently shrink and 

 become vaguer of outline. We often happen to see specimens which have become crooked 

 and irregular from pressure, otherwise all the animals are naturally symmetrical. The 

 size of the adults varies considerably ; in most species the diameter of the animal seen 

 from below is 2 /b — lVzmni. ; it can even decrease to about '3 mm. (Romoeoscelis mediter- 

 ranea), and Choniostoma Hansenii G-. and B. can obtain a lenght of 5'3 and a breadth 

 of 5 - 5 mm. As a rule there is a certain proportion between the size of the parasite and 

 that of its host; however, it must be borne in mind that the parasites themselves differ 

 in size according to their habitation in the marsupium or under the carapace; in the latter 

 place they are comparatively smaller. As a matter of course, small Amphipoda cannot 



