754 report — 1884. 



In the Peruzidea the young is supposed to he hatched in a nauplius form, which 

 has heeD demonstrated in Leucifer : in Sergestes and Penteus, as has approximately 

 been shown in Sergestes, it is produced in the form of a blind Xelaphocaris. In this 

 division the ova are not connected with the animal by any membranous attach- 

 ment, and are probably deposited in the open sea, there to be brought to maturity 

 if they do not fall a prey to hungry animals. 



Within each division a greater or less departure from the typical character 

 takes place, the variation generally being the greater or less developed condition 

 of the branchiae and the pereiopoda or walking legs, and this departure is generally 

 connected with an important change in some external feature belonging to the 

 animal, although there are some instances, as may be seen in the European and 

 North American crayfish, where there is an alteration in the branchial condition 

 without any external evidence. Sometimes, as in Notostomus and Miersia the 

 branchial arrangement continues the same, while the external features vary con- 

 siderably. Again, in the history of development, as in the genus Alpheeus, the 

 young of some species are hatched in a distinct form from that of others, while the 

 parents exhibit no sign of variation, while again the common shrimp and the 

 common prawn (that is Crangon and Palcemon), which evidently differ "from each 

 other, hatch their young in the same form. 



The variability according to its importance and constancy breaks the several 

 divisions into Families, Genera, and Species, while the character of development 

 demonstrates a continuity in the history that exists in apparently widely separated 

 species ; an example of which may be seen in comparing the genus Ibuccus with 

 that of Palinurus, which appear to possess little or nothing in common, while their 

 offspring in the earliest stages of development appear to be identical. 



But in whatever stage the young may quit the ovum, there is one common 

 chapter in their history ; that is, they invariably seek the surface of the sea, 

 feeling as it were for light, air, and warmth, which for most of them would 

 appear to be the chief condition for which they struggle in the earlier days of their 

 existence, after which they generally sink to that abyss, whence they have been 

 brought to light through the far-reaching powers of modern investigation. 



Animals so delicate and fragile as these are very liable to injury and destruction 

 from many causes. 



As far as our experience teaches us, they never in the rougher weather reach 

 the surface, or, at least, remain there, and where they have the power of congre- 

 gating most they fall a prey as food to many larger animals. Taking their several 

 means of destruction besides those that seem natural to the condition of their young 

 lives, we must suppose that a very much larger number must be hatched as com- 

 pared with those that permanently arrive at the adult form. 



It is difficult to determine the number of 3 T oung born of any known species, but 

 it must be very great if we consider that a prawn of average size bears about a 

 thousand eggs, and that a large Palinurus, such as is sold in our markets as the 

 crawfish or spiny lobster, produces many thousands ; the ova being small, or about 

 one-fiftieth of an inch in diameter, while those oiHomams are one-tenth, and those 

 of Astacus are one-eighth. 



The stage at which development of the embryo has advanced at the time when 

 it quits the egg appears to bear some relative proportion to the size of the ovum. 



Thus our common edible shrimp has an egg only one-fortieth of an inch in 

 diameter, while that of the Arctic species has one that is the eighth of an inch. 

 The smaller ovum produces an animal in the zosea condition, while from the large* 

 the young appears in the megalopa stage. 



Palinurus, Arctus, Ibaccus, and other allied genera, produce their young in an 

 intermediate condition, more advanced than a zoaea and less mature than a mega- 

 lopa, in a form long believed to be an animal of distinct individuality known as 

 P/iyllosoma. 



The genus Palinurus, perhaps more than any other, will assist in demonstrating 

 the bearing which the character and condition of the young have in the geographi- 

 cal distribution of species. 



I cannot here resist pausing to remark that, if an animal that has furnished 



