NOTICES OF NEW BOOKS. 155 
earliest division of the yelk to the period when it leaves the egg 
is briefly sketched out. 
In his next chapter, Prof. Huxley treats of the comparative 
morphology of the Crayfish—in other words, its structure and 
development are compared with those of other living creatures. 
After a long description of the common English species of Cray- 
fish, the terms “species,” “genus” and “family,” used by 
naturalists to denote the various groupings of animals possessing 
a common plan of structure, are explained, and it is shown 
how all the different kinds of Crayfishes may be regarded as 
modifications of a common Astacine plan; that in a similar 
way all the Arthropoda are connected by closer or more remote 
degrees of affinity with the Crayfishes, and that ultimately all 
living forms may be regarded as related to one another as being 
either cells or composed of aggregates of cells. 
The author's classification of the Crayfishes by the pecu- 
liarities of their gill-structure is almost entirely original, although 
Erichson and Dr. Hagen had previously separated the Eastern 
and Western American species by a difference in the number of 
their branchie. The results of Prof. Huxley’s researches have 
already been brought before the scientific world in his paper on 
the classification and distribution of the Crayfishes, published in 
the ‘Proceedings of the Zoological Society’ for last year; but 
they are exposed in a more popular manner in the present work, 
and characters of so much value to the student of the evolutionary 
history of the existing species have been discovered that we 
doubt not, when new workers have been attracted to the subject 
and the study has been pursued more into detail, important 
modifications of the systematic arrangement of the larger groups 
will result therefrom. In the Astacina, at least, good characters 
have been discovered for the definition of forms widely separated 
in geographical position, but hardly to be separated as species— 
certainly not as genera—by external characters. 
The distribution of the Crayfishes over the surface of the 
globe, and the correspondence of their structural differences with 
the peculiarities of their geographical range is considered in the 
final chapter; each species, genus and family has its peculiar 
distribution, and that of the group as a whole is compared with 
the analogous distribution of the fresh-water Salmonide. All the 
Crayfishes of the Northern Hemisphere are shown to belong to 
