178 INVEETEBEATA chap 



internally all the blastomeres merge in an unsegmented mass of yolk. 

 If this superficial segmentation occurs only on one side of the egg — 

 i.e. if the daughter nuclei migrate only to one part of the surface — 

 the segmentation becomes meroblastic, although, as we shall see, 

 this meroblastic segmentation differs most markedly from that found 

 in Cephalopoda, which is derived from the telolecithal method of 

 development. 



ASTACUS FLUVIATILIS 



We select as type of Crustacean development that of the common 

 river crayfish Astacus fiuviatilis. A full description of the develop- 

 ment of this form is given by Eeichenbach (1888), and no such 

 thorough account of the development of any other form has been 

 given before or since. 



As every one knows, the eggs are carried throughout their entire 

 development by the mother, attached to her swimmerets by a glutinous 

 secretion. Eeichenbach found that when the eggs were removed 

 from the parent they quickly degenerated and died. In the case of 

 the allied genus the Lobster {Homarus), it is perfectly feasible to rear 

 the eggs after they have been removed from the swimmerets of the 

 mother, but in order to do this an elaborate arrangement must be 

 provided so as to secure a constant supply of fresh aerated water to 

 bathe the eggs and to ensure that they shall be constantly agitated. 

 Such an apparatus is provided in the various lobster hatcheries 

 built and maintained by the Canadian and United States govern- 

 ments. In default of such apparatus the plan adopted by Eeichenbach 

 seems to be efficient and simple, viz. to keep a large number of 

 females carrying eggs in an aquarium, and from time to time to 

 remove a portion of their brood for examination. 



By means of very simple arrangements the females can be kept 

 for a long time in a state of perfect health. If the bottom of the 

 tank be covered with only a few inches of water, and provided with 

 an overflow ; if a slender stream be kept constantly falling into the 

 tank from a tap; if the tank be provided with a covering of wire- 

 netting in order to prevent the crayfish escaping ; and if the whole 

 tank be kept shielded from direct sunlight — then all the conditions 

 will be fulfilled necessary to maintain the crayfish in a healthy 

 condition. They are easily fed on earth-worms, scraps of fish, etc. 



All the eggs belonging to any one female are in the same stage of 

 development at one time, but the period required for complete 

 development is a very long one, extending over several months. 

 Thus, by keeping together a large number of females with eggs in 

 very different stages of development, a complete series of stages can 

 be picked out in a very much shorter time than would be required 

 if the eggs of the same female were taken for all stages. 



The eggs of the crayfish, like those of most Arthropoda, are very 

 difficult to deal with, as they are composed chiefly of semifluid yolk 

 enclosed in a very tough resistent membrane, and if an attempt be 



